Hi,

I already have set up RouterBoard 750, but at this moment it works with one static ip in one subnet, however I need 2 static ip in one subnet using one DHCP server.
Is there any way it can be done? Maybe it's better to use 2 RouterBoards?

I think your solution is to divide your subnet in 2 parts and add src-nat rule for each with different ip, for example LAN 192.168.1.0/24. For 192.168.1.0/25 src-nat to one ip and for 192.168.1.128/25 src-nat to second ip, or use address list.
Hope will help!