> [...] Is there a way to give both systems an open NAT at the same
> time?
Sure, _if_ you can have them use different ports.
> [...] seems like the uPnP should take care of it automatically [...]
How? Port forwarding and UPnP are not magic, and the router lacks
intuition. When a message arrives at your router (from the outside
world) for any specific port, the router must decide whither to direct
that message. It can direct it to only one place, not two.
If you can tell the fellow at the other end to use a different set of
ports, then you can use explicit port forwarding or UPnP to direct a
message for those different ports to some different destination (and,
perhaps, to different ports from the ones in the original message).
