Would be best since it would then match. The only other idea would be to create some effect at the edges of both wallpapers (maybe a glass shattering or blurr) to link them if you do different colors. Anyway Yay!!!
Actually doing a set using all the 'nation' vectors (Changelings, Equestria united, ect) in Emkays gallery would be awesome so long as they match. I've always been a bit disappointed in how few multi monitor pony wallpapers there are (Although usually you can find a couple that fit one another thematically)
Theres a changeling logo in Emkays gallery called the Changeling Swarm [link] Looking great dude. I do have skype but right now school is stopping me from doing much of anything for the next couple hours