If two identical radios are side by side and tuned to the same frequency, will they both pick up the signal at 100%, or will they wrestle for the same radio waves?

Imagine you are with a friend on the beach., side by side on the water and a big wave comes. Do you fell less pressure because your friends is by your side?

Well yeah. That's one of the benefits of a good friendship.
How friendly we talking about?
- Strong force friendly
If they're really close to you or the waves are small enough to block, yes. Otherwise, no. It's a great analogy.
whether you feel less pressure, or whether there is less pressures, are two completely different things. the question was about 100% reception, and it's fair to ask, so far, no one's answered it, you know, scientifically.

Ham radio operator here: basically neither will happen because both don't really mean anything.

This is an imperfect analogy, but I think it will set you thinking in the right direction: If someone is blinking a flashlight at you, and you're sitting right next to another person, do both of you see the flashlight at 100% brightness, or do your eyes wrestle for the same light waves?

What does "pick up the signal at 100%" mean? Let's say me and my buddy are talking on our car radios, no repeaters just point-to-point. If we start off in the same parking lot, we can easily hear each other. If we start driving in opposite directions, we'll still hear each other just fine, until one of two things happens: We go on either side of a hill or far enough to be beyond the horizon, and then abruptly stop hearing each other, or the signal will fade in intensity until the background noise is louder.

If we get to that point where the signal is weak but still receivable, increasing output power of the transmitter, or switching to a directional antenna might help. People tend to think antenna gain is some magic that makes the radio louder, but it's not. A high gain antenna does the same thing that cupping your hand behind your ear or around your mouth does; it puts more of the energy that would have gone in different directions in the direction you need.

Without getting too far into antenna theory, I will say that yes having two antennas near each other can cause them to interfere with each other. "Wrestle for the same radio waves" isn't the way I would describe it. Antennas resonate with radio waves, it's like a tuning fork, if you play the note the tuning fork is tuned to, the tuning fork will start to vibrate and emit its own sound. If two antennas are quite close together, this can cause destructive interference. You can use the same principle to construct a high gain antenna; look up how yagi antennas work for more details.

If you don't mind a followup question, what's happening when a signal clears up if you touch or just hover near an antenna?
- possibly several things but my first thought is your body is acting like a capacitor to ground. I'm guessing you've noticed this on an FM radio or rabbit ears on a TV that probably weren't grounded well.
- That can be for a few reasons...
  
  In some cases you're tuning (or detuning) the antenna capacitively.
  
  On other cases, like if your tv gets interference when you're standing in part the room, there may be standing waves causing interference, as the rf is bouncing around your room.

Radios receiving signals don't just siphon the signal off lol

What you're asking would only really happen with wireless Internet service and it's not because of the wireless signal, but because the overall bandwidth diminishes the more people connect to it.

I mean, literally there has to be at least a tiny amount of energy transference right?
- It’s like solar energy. You either absorb it with a panel, or it goes to “waste”. You’re not really stealing it from someone else, as long as you’re not getting too much in the way
- Actually, the waves emitted by the radio tower are enough for a receiving device to generate a small electrical current just through the oscillations of the propagating signal.
- Yup. It's typically amplified quite a lot in the receiver, and the vast majority of power transmitted never is received, so it doesn't usually matter, but it's not a dumb question.

How side-by-side are we talking? If the antennas are closer than their size, yeah, it won't necessarily work the same way because they'll act like one antenna. If they're too far apart for "near field" effects (or if your antenna was tiny relative to the wave to start with, like with AM radio) it won't matter, because the wave in question will just kind of ooze around any obstruction, and received power will just go with inverse square of distance to source again.

In practice, it's unlikely to matter so much how loud the signal is, because (unless you're using a crystal radio) you are definitely going to amplify it quite a lot before it's useful, anyway. More of concern is how loud it is relative to any random noise that's present, which is not so dependent on antenna area.

Edit: I suppose if it's between you and the source, it will dim the signal a tiny, tiny little bit. Not the way a bigger thing can cast a shadow, though; think more like a slightly dirty lens.

Depends. If the antennas were resonant dipoles placed some fraction of a wavelength away from each other (1/4 wave away), you may get some cancellation of the signal.

Look up the "yagi uda" antenna, it's the classic rooftop tv antenna. The elements are spaced by fractions of a wavelength to achieve directivity. One single element is driven, the others are just resonant lengths of wire.

A laymans opinion on the challenge: Waves lose energy, and the exact placement of antennas will matter. I don't know what the mechanism is called, but we don't place wind turbines right next to each other. That is afaik because each turbine takes some of the energy out of a larger chunk of the wind-wave in an 'bubble' around it, so we place them with optimal distance according to efficiency of that mechanism. If I'm right the effect will probably be minimal. Anyway, just a stab at an interesting thought..

Yep. It's called near field and far field in radio. In the far field you can approximate it as a beam from the transmitter, while in near field it's magnets and things can absolutely interact. You never want to put up a stand-alone antenna in the near field of something conductive. Those big tower antennas actually incorporate the ground as a critical part of their design, because of that and the non-negligible conductivity of ground water.

People are answering what you asked but what you probably mean is in a 2d world where two antenna are perfectly in line with the transmitter will the first absorb some of the signal - yes it will, just like two wind turbines in a line it's absorbing the energy from the medium and using it to do work.

It's not always so simple, it might spit some of if out too if it doesn't have anywhere else for it to go and it'll do this in a certain pattern which can, depending on the distance and arrangement ,increase the signal received by the second one. This and similar principles are why you see so many odd shapes for antenna designs such as the many bars on a TV antenna which make it more directional.

You and the person on a chair on the beach next to you will both get sunburnt. In the same way, radio waves washing around your house or car interacts with everything, antenna or not.

IF's might heterodyne