If you still haven't got that Dynamic Duo achievement then please let me know because I'm after it to. I'm not looking to co-op play beyond getting that achievement though because at the moment I'm enjoying seeing just how far I can go solo.
I don't usually bother with headsets either as I'm not normally one for online play but that shouldn't matter though as this achievement is very straight forward. The person who powers up their melee attack first is the one who gets the achievement. I'd figure stage 1 on chasm in that opening room with 2 Oozes should do the trick.
Let me know pal if you're up for it. If I don't hear back then I'll assume that you've already gotten the achievement by now.