Just to be clear--you won't connect to them, they'll connect to you. You'll launch the campaign, and they'll join your game. Are you saying only one of them can connect to you (either via direct IP or an alias) but the other can't? If so, start with JohnD's question above:

- What does it say in the upper-right corner next to the version number?
- What happens when the second user tries to connect to you (error, etc)?