Help with Router, 2 internal interfaces
 

Hi, I am setting up an openbsd router/gateway. It will serve as a gateway for 2 linux machines. I would like to set it up like this:

Router
|- xl0 ----- Internet
|- xl1 (192.168.0.1) ---- Internal machine 1 (IP: 192.168.0.73, Subnet Mask: 255.255.255.0)
|- xl2 (192.168.0.2) ---- Internal Machine 2 (?)

It is working fine for internal machine 1, and I know that the second internal machine must be on a different subnet but I don't know how to set it up. I'm pretty sure the second internal machine should recieve an IP from a different block (10.0.0.x or something) But, what subnet masks would be appropriate for the internal interface on the gateway/router and the second internal machine? What would I need to do to allow the 2 internal machines to connect to each other? Any help would be greatly appreciated.