The number of servers really isn't relevant. The problem is the number of Exchange organisations. You can have many exchange servers per organisation, and IIRC one organisation per AD forest (e.g. AD domain or related group of domains). You can have many entirely separate AD forests on a 'physical' LAN and each one of those can support their own exchange organisation, of course. It's written for exchange 2007 but you may find this Microsoft technet documentation helpful to understand these concepts and terminology.
Each organisation can support multiple domains (both in terms of email and AD domains), and you can choose to have multiple servers in your organisation with separate servers housing mailboxes for each mail domain (or AD domain or whatever).
So yes, what you're doing is possible. Your next steps to implement this depend somewhat on what exactly you mean when you say you want to add a new exchange server "to the network". If you want a test environment for development or to rehearse a migration then I strongly suggest implementing a totally separate AD forest/domain on a virtual server to install Exchange 2010 into.