Here’s a great article for you to read.
it’s quite simple though, it is mainly wizard driven. If you installed the first one, then you will be able able to do the second one. All you need is to specify, before installation starts that it’s an ADDITIONAL EXCHANGE SERVER.
That’s all, it will do the rest. it then uses your First EXchange server as an SMTP relay host. You may need to add the new one’s IP address to you SMTP relay access on the First. It does not need any special configuration to be connected to the First.