I'm the creator of "Wings of Liberty Coop" and me and my team have now also published "Heart of the Swarm Coop" on the arcade on all 4 regions (EU, USA, Korea, China)
Just like in the previous campaign, the maps support 2-3 players who can play through the campaign with their own hero and evolution choices.
So if you wanna check it out on the arcade, just search for "Hots Coop".
Cheers!