What a strange idea to devide your users in 2 groups !
For your question, what happen if you bind the 3th server on both 2 OD ?
Probably that they have the same range of UID, so in some cases (for ex with nfs) 2 users will access to the server with the same credentials.
Ultimately you don't as one OD server isn't going to know about the other. Given your implementation you're bascially stuck with going with a public folder that's available to all… and I mean all. I typically prefer an OD master/replica solution where both servers have a full copy of the OD service and the services themselves are seggregated between the servers as you choose.