Yes, that's possible, if you use a sophisticated Version Control system, SVN, Perforce or such, and both developers have the license for GG and all assets used.
Still you would have to synch regarding the committs and checkouts in the repository and ensure that NEVER EVER you both are working on the same FPM map.
But, actually, for all assets and scripts and working on different levels at the same time, it would be absolutely doable.
Here a very basic and simplified Introduction to Version Control.
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