You can have them installed on 2 macs - but the license prohibits 1 license from used at the same time!
If you want another CS5.5 licence then check Amazon or other reliable online stores.
Doubtful. Adobe is not selling CS5.5.
That license only allows one user to access two machines. If you have two users or the machines are used concurrently you are violating the license.
But to answer your question, CS5.5 is not available through any legitimate channels that I know of anymore.