I need to completely open some ports for Xbox Live. I've got two Xbox's that I play regularly and therefore cant forward them.
Ports are 88 (UDP), 3074 (UDP), and 3074 (TCP).
How do I open them?
"Opening" ports on the SME server (which is connected directly to the Internet) won't achieve anything. It is not interested in those ports.
What you want to do (I think) is forward those ports to the Xbox which is interested in them.
As you rightly say, you cannot forward the same ports to more than one box. "cannot" means exactly what it says - it cannot be done. Incoming traffic on ports 88 (UDP), 3074 (UDP), and 3074 (TCP) must be sent to one of the Xboxes - it can't be sent to both of them.