Martin's answer is correct.
Configure both mail servers to accept mail for your domain, and for their own unique hostnames.
e.g. if your domain is example.com and your mail servers are mail1.example.com and mail2.example.com then mail1 should accept mail for both example.com and mail1.example.com and mail2 should accept mail for both example.com and mail2.example.com
Note that mail1.example.com and mail2.example.com need to be defined in your DNS or as entries in /etc/hosts on both servers. Presumably, they are already defined in DNS because you have MX records pointing at them.
Entries in /etc/aliases on mail1 would then redirect mail for some users to mail2 - e.g.
Similarly, you could also have aliases on mail2 to redirect mail for some users to mail1 - but be careful to avoid creating a mail loop if both servers define aliases for the same address.
Don't forget to run the 'newaliases' command after editing /etc/aliases.
On a more general note, if one of your mail servers is running postfix and the other is running MS Exchange, then hide the Exchange server behind a firewall and make the postfix server your mail gateway. The postfix server should have the sole MX record pointing at it, so all incoming mail goes to it. It then decides whether to deliver inbound mail locally, or forward some of it on to the exchange server.
You almost certainly do not need two MX servers, and having two will likely cause you far more trouble than you think it will solve, especially if one of them is a capable, secure and robust mail server like postfix and the other is Exchange.
For a brief summary of some of the reasons why having a second MX server is usually a mistake, see:
That question was specifically about backup (lower priority) MX servers, but the same principles apply - with added complications because (due to DNS round-robin) roughly 50% of incoming mail will go to one server and 50% to the other.