You can actually register in both, which is useful for local elections where you can vote in both. But in a general election, you can only vote in one. I assume that you can vote in any byelections.

Why's the text on that site so enormous? I can only see 5 lines at a time on my 17″ screen.

Similar. When I played it, it felt like it had lots of bells and whistles whose main contribution was to make it more difficult to learn. Possibly some people just like complex rules and the process of learning them.