Check FCOM 2 for what causes each of the lights. I'm not sure 'Loss of Both Gens' is relevant to the question; they are independent its, so the relationship between their warning lights is the same L & R. I can see that Transfer Bus Off would relate to Loss of Both.
DRIVE lights monitor the oil pressure, temp and rotation of the Gen. Source monitors the electrical output of the Gen. The former is a mechanical issue, the latter electrical. A Gen could fail for various reasons. An electrical failure doesn't necessarily mean a mechanical one, but a mechanical one will cause an electrical one. Checking the lights may give you an insight into which type of failure you have.