You can't active them at the same time, even you can (by forcibly start gateway), the comunication with Agent might be messed up . I don't even understand how you installed two Gateways on a single server, shouldn't be second installation be an uninstallation?
DM Gateway actually can talk with Agent in different subnet, no matter it's behind NAT, in DMZ or not. So may I know why you need the second gateway?