Based on the schedule on the website, 9:10 on the ground isn't long enough for a layover once you include brief and debrief. I don't know what Delta uses, but that eats 1:30 of the layover at CAL.
With another flight or two that stop at Sal Island the trip would look like (2 crew):
ATL-SID
SID-MLW-SID
SID-ATL
With 24+ hours in SID. With one more Sal Island flight the trip would take 8 days to complete, with 2 more it would be down to 5 (one) or 6 (two).
If there arent anymore Sal Island flights then they probably would do (3 crew):
ATL-SID-MLW
MLW-SID-ATL
with a 7 day layover in Liberia, for a total of 8 days away.
It would be cheaper for the company to put on a third pilot than to pay the trip rig for a 2 week trip with 2 pilots.