When you're only running two instances, I would wait with the load balancer and start by dividing up server roles. So you run the PHP application on one server and the PostgreSQL on another.
You will get more efficiency, since it's easier for a server to always do the same task, than it is to jump between the database and the php scripts. Especially for the database.
When the time comes to add the third server, you can add a front end server and set a load balancer between those two, but both of them will be connecting to the same database.
But there is another issue with the load-balancer scheme which you didn't mention, but you will definitely run into and that is maintaining sessions. But if you don't have logged in users, that's probably not a problem for you.