It is strongly recommended that you use the exact same hardware and OS configurations on both machines. This includes firmeware, drives, Windows Service Packs, Windows Security Updates & Exchangee SP/Security Updates.
Keep in mind that you will need seperate storage which both nodes can access without dependancies of the other node. It can be SCSI attached, NAS or SAN. Look into which approach is more cost effective for your needs and good luck. You can find more info at www.microsoft.com/exchange.