Synchronizing two pistons

I am trying to get two pistons to synch their pulses for an item sorter (nether).

In the attached pic:

1. Observer which has a string in front of it (far side) that sees items drop down out of a cobweb, it pulses the redstone to start the pistons
2. The primary piston (#2) that pushes the items a block over towards the secondary piston (#3)
3. The Secondary piston (#3) that pushes the items along the ice path.

The redstone and repeaters are currently set up so that the observer pulse pushes the primary piston first, then after a short delay pushes the secondary piston.

What I am trying to do is prevent the primary piston from extending while the secondary is active. What's happening is every now and again due to timing of items dropping past the string, the secondary piston is extended and the primary piston pulses pushing items onto the top of the extended slime block attached to the secondary piston, vs. those items dropping into the empty space while it's withdrawn.

Any help appreciated!