Thank you Myfist for taking the time to help with my issue. Below are my responses in bold.
I run the Steller Phenom and Forbidden Worlds. I also have the SoA2 maps installed, but only on my CPU not my brothers. Do you recommend not running any of these?
Up to date on Steam
Both or none are on the Sins public beta.
Not trying old saved games
Just out of curiosity, do you join over ICO or LAN/IP connect?
Running on ICO. The checksum error when he trys to join my hosted game is "The mods checksum differs from the hosts. Ensure that you are running the same mod and version."
He just doesnt connect when we try LAN/IP, but that may be a port issue that I havent further explored as of yet, as I am behind a router.
It has been a long time since I heard of a connection issue, did you install from the same download? it is not uncommon for modDB DL's to get a little borked.
I installed from the same link from moddb on both CPu's.
2nd link down.