It will have only 8 (outgoing) connections.
With fewer connections, it will notice new blocks a tiny bit more slowly than the other machine, and blocks it finds will propagate to the rest of the network slightly more slowly than the other machine. So it will spend a teeny-tiny bit more time working on an out-of-date block chain, and will be a teeny-tiny bit more likely to lose "announce a new block" races. But I bet the effects are so small you'd never notice them.
Can't you eliminate this effect by starting Bitcoin on the second machine (not getting the port forwarding requests) with the
-addnode= parameter ?
In that way, this second machine will have 7 connections to the outside world and 1 to the other machine probably being connected to a lot of more nodes.
I think even using the
-connect= on the second machine is enough as the connection between the two machines is probably 1000x faster and less laggy then the internet connection to other nodes.