Read these instructions first: HOW TO PLAY MULTIPLAYER
You cannot play DarthMod with your co-player online?
(Launcher version) Ensure that you both have exactly the same options. Recommended is the MP Setting and (optionally) any other from the "Graphics & Sounds".
(Steam Workshop Version) Be sure to Disable the Launcher version of the mod (if you also have it) and clear any other mods that may interfere.
You cannot join Vanilla Multiplayer battles?
(Launcher version) Press "DISABLE TO PLAY VANILLA" in the launcher.
(Steam Workshop Version) Remove the sounds_fixed.pack file from your Data folder usually in C:\Program Files (x86)\Steam\steamapps\common\total war shogun 2\data