Yes, me and my partner do it all the time. We work off a Master Assets.esm,. i.e. my mod. We work each separately on different things and from two separate .esp's.
Reason why we work from an .esm is because when we work we need both FalloutNV.esm ticked, and the VaultX6.esm ticked, our mod.
Then if we're working from scratch esp's, which we do every time we make a public release, we name the esp's with our name, the start date, and the end date.
The reason for dates is so we don't get outa sync and horribly screw up our mod. The start date means nothing, but the end dates have to match, or we're out of sync. So if your currently working on a mod, DL Fallout New Vegas Plugin, load your main mod in it, select edit, then toggle master. In the white box that filled up with text when you loaded your mod should now say master at the top of it, with numbers next to it. Save as (so we make a backup just in case), then go to your data folder. Your mod will still say .esp at the end of it, so rename it MyMod.esm, this will effectively make it a Master file. Note: Changing it just simply to esm doesn't work, your still have to change the file inside with FNVPlugin, or GECK will still load it as a plugin.
K so now we have our esm. So load up GECK loading FalloutNV.esm and MyMod.esm, select a Cell your gonna edit, start making changes, I suggest just making one change so we can save our new esp.
Just move a box, then click file and save, then save your new esp. If you have a partner like I do, me and tananbaum found it simple the way we do it. We have VaultX6.esm, and we have two esp's I merge in to the ESM on a regular basis, everything 2-3 days, Brad1_19_1_21.esp (mine) and Chris1_19_1_21.esp (Tananbaum). So this is what we're working out from right now, we started work on the 19th, and We'll be making a release this friday. So this friday he'll send me his esp then i'll take Fallout New Vegas Plugin, load my esm, click file, merge plugin, then I would select Chris' esp and merge it, save the esm, then do the same thing and merge mine.
So now I'm sitting here with a new esm, and the easiest way you can tell this is that the new ESM is bigger, and when you load it, the new content IN the esm, without any ESP's loaded wont have * next to it.
So then I send him the new ESM, we create two new ESP's, i.e. Brad1_21_1_22.esp, Chris1_21_1_22.esp, and do the process all over again. And just so we know we don't mess with each other's work, the Maps have our own special code in the ID, and we just don't mess with each other's cell's... until I start cluttering them.
http://www.newvegasnexus.com/downloads/file.php?id=39655