Not a very timely reply, but this might help.
Assuming you already realise that you need to use a single backend database ;) the question is going to be about nsv and similar shared data. OpenACS makes use of AOLServer's excellent thread safe shared data structures. Obviously these are local to a server process.
You can mitigate this issue by using what most load balancers call "session affinity" which means that if the same person (identified by a cookie etc.) hits your site twice they will always get the same server (unless it goes down).
Or you could suck it and see, but being aware there would *definately* be code changes you would have to make.
