They can be on the same major net (eg 192.168.1.x), as long as they are on different subnets.
For example:
ip address outside 192.168.1.1 255.255.255.240
ip address inside 192.168.1.65 255.255.255.192
Note: if that router is randomizing sequence numbers (which I doubt) and you are using statics, you may have to use the 'norandomseq' keyword or your data may get scrambled.
Hope it helps.
Steve