I've not done this but you can use Nginx to load balance. The principle being that you have a single entry point which decides whether to send the traffic to your vps1 or vps2 server. This can either be random or round robin, possibly even user session based.
YetiShare already stores PHP sessions in the database, not locally on the server, so I can't see this being an issue.
The main problem will likely be access to the uploaded files. If a file is uploaded to vps1 and the load balancer sends download traffic to vps2, it wont find the file. You could try running rsync via a cron task on each server to make sure all the uploaded files always match.
Just throwing a few ideas out there at the moment, as I say I've not tested it load balanced unfortunately.
