Dolphin, the GameCube and Wii emulator - Forums

Hmm, not sure what's happening here :/ Try the latest development build (make sure your friend gets the same build as well). If that doesn't work, maybe try creating portable.txt in a copy of your Dolphin folder (again, make sure your friend does this too) and try running netplay again.

Can you try the latest development build and see if it happens there? Is there an older build where this works?

Mario Tennis doesn't support deterministic dualcore. Use Single Core.