I have seen no problem when using my two OBi110s or one 110 and one OBi1032 phone on the same subnet.
For most of the port numbers in that FAQ the instructions are to ensure that the outgoing ports are open. Port opening is not the same thing as port forwarding. None of the tens of home routers I have examined had end-user controls for opening outgoing ports. Yes, virtually all home routers can be configured to do port forwarding but port forwarding affects only incoming ports.
The only incoming port mentioned in that FAQ is 10000 which is used by Obitalk. I've never had to do anything with that one, even for my remote OBi100 on the other side of the world behind an ISP's Carrier Grade NAT and a home router's NAT.