They would see both caching servers as long as they are set up similarly and are serving the same public IPs. The way it would work would be, they would both have the same "rank" as far as the client was concerned. A client would query the caching server with the lowest rank, so in this case it could be either, then if the caching server did not have the content it could look at its peers (you should set up peering) and if the other server has it, it can serve the client. If neither server has it, one would download the content on the client's behalf and serve it to the client.
If you have one server set up for a month or so, you can see the performance indicators in Activity Monitor.app. Look at the pressure metric. If that is much above 20%, then you might need more caching storage (more disk space). More caching servers would help keep the pressure lower, if that it even an issue for you. If you don't see much pressure at all, and your metrics are not going crazy in Activity Monitor.app, then one might cut it.
