Yes port forwarding is a 1 to 1 arrangement. UPnP is just a way to configure temporary port forwarding rules, albeit in a highly insecure manner that threatens your whole network. I would never advise using UPnP at all.
Either way will not work, as the External port remains the same. When a connection request comes into the router, it will either block one of the rules & send everything to the other console, or you have no idea which one it will go to if you get both enabled.
What you need to do & this is dependant on how much control you have over the destination port used for the inbound connection from the internet. Anyway, create one PF rule for the original external port as you did before pointing to one console. Then create a second rule, use a different external port, but mapped in the rule to the same internal port as the first rule. Point this to the second console. Then you have to make the incoming connection from the internet for the second console use this new port number.
