You can't combine the same interior cell from two different esps. It will use the one which has been edited last.
Also, for exteriors, when merged it will take the latest edited esp's land, and if ANY objects are different between the two esps, then it will add those as well. If objects are exactly the same, and in exactly the same space, then these duplicates will be removed automatically.
So it's best for only one person to touch the exteriors, doing the land raising and object placement, otherwise you'll end up screwing it up and it will get difficult to fix. And for each person to be allocated interiors which they do, and no one else touches.
Good communication and organisation is key.