This is an interesting idea. There are so many things to think about when you get 2 or more people working on something.. when those 2 people have never met (which may or may not be the case)
Obviously all parties involved have a love for gaming... but even with that common love there is a lot of variance on how that love translates into creative projects. The biggest hurdle I see is what happens when and if you guys disagree on what the maps have to have to make it work for a regular DnD game vs a 4E game. Obviously the idea is to be generic enough that it ok for any scenario.. but that may lead to very generic maps.. maybe not. I've played 4E for several months now and honestly I'm not sold on it.. but one thing I think that it needs to be interesting is interesting battlemaps, things that make the party approach encounters in different ways. It's not a matter of your abilities, obviously you are more than capable of making great maps. It's the difference in the game mechanics.
I'm not very good at making points.. so let me explain my background in a similar or not-similar situation.
Four Ugly Monsters was originally set up as four guys that were going to be making modules for gamers. we had 2 cartographers, 1 writer, and me (artist, token maker, minor cartographer) at the time we all enjoyed 3.5 DnD (4e hadn't even been whispered about yet) We had never worked together before forming FUM. The 1st obstacle was figuring out the name and look of our group. That alienated one guy and he quickly within 2 weeks of starting up dropped out. The remaining three started work on their first project and almost completed it before the writer lost interest and didn't finish the story... every project he started though was for his personal game and needs and he couldn't find time for other peoples ideas. Later that year he quit after the other guy got in a little flame war with another publisher. Now we were down to two.. I started making tokens for myself and he made map packs. We had a store and sold my tokens and his map packs.. My tokens sold 5-6 times more than his maps, but our pricing structure gave him and the writer for a while a percentage of the token sales. Also a certain percantage of sales went to hosting and software purchases. So while I was producing the most work, selling the most material and keeping FUM alive the writer guy was off doing his own thing and the remaining cartographer was slowly losing interest in any specific goal and just did a lot of random things. He later dropped out and I was left holding FUM and looking back at all the projects we talked about that never saw the light of day, and all the money they got from my work.
Now we were a bit younger and not as well established as the three of you. But figuring out a pricing strategy and having plans in case one guy drops out or fails to produce his part is a good idea. What label are you putting on the packages, who gets credit for the idea, How is the money collected and kept, and then divided.. I was lucky in that I was the guy with the paypal account and I decided when to pay the other guys, so that when the time came I could cut them off and all future revenues were in my hands. You'll want to know and have access to the sale numbers and the income even if you aren't the guy collecting the funds.
I know this sounds negative.. but it's not and I'm not even bitter about the above situation.. it's hard to work long distance with people. You are fortunate that these guys are established and can't just fade into the woodwork and turn their backs on the job. I expect that you will have a different experience all together. And I hope it is a successful one. If this does work.. I'll find a way to make my next endeavor work better because of it.
Looking forward to watching this progress and seeing the products.