M Emile
When mappers colaborate a map, if mapper1 has the main file can mapperB create a part on a different .map and paste it into the main one?
You have another option of what WSike said.
Its a bit hard to explain but i will try xD.
Basically you can hold Shift and with left click (select) you select the layers u used for that part and then drag your mouse to copy the whole part. Then without clicking anywhere else and having the whole part selected. Load the main map and you select the same layers and then just click to place wherever u want it (edited)