I am just a little curious how 2 people make a map together.. being online at the same time and working on it at the same time so that they can talk together.... maybe you have it figured out.. just seems like an odd way to make a map to me.
Me and Lava Croft are making a map together.
I do the human end, he does the Alien end and then we stick them together.
No colaboration, no discussing either's maps.

Having two people doing a map makes the design style more interesting, as it's much easier for someone else to say 'That's crap, it's all rectangly' than it is for you to think about it.
Having someone constantly critisising the map forces you to build it at the best you can. :3