You can only set up your NAT to port-forward to one of the computers.
For redundancy in case the first computer goes down, you could have two that connect out and the rest connect to both of them. The first two are run normally, the rest are run like:
bitcoin -connect= -connect=
I agree with the first statement (you can only forward the port to one computer).
Therefore I do not understand the second statement. How does this achieve redundancy? Only one of the two will receive forwarded traffic. If that one goes down then you've lost the connection (AFAIK).
Am I being dumb?
I can think of how it can be redundant, if the second connect command is connecting to a known, usually running, node Outside your own network, a public IP address. There, redundancy achieved.