The only reason to have to MLGW's on one network is if you're using a single room lighting solution (ie. Lutron Grafik Eye's not linked together). Other than that (especially once the ability to have multiple users was added) there's really no reason for more than one MLGW. Unless of course you don't have a way to run ML between the two systems and you need to be able to use the iOS app...
Also, in case you actually want the answer to your question, the login credentials for both gateways needs to be different and you'll need to manually switch back and forth between the two in the Settings menu of the iPhone app. You cannot just duplicate the same config for both gateways. Not convenient at all and in most cases not necessary.