To divide one's attention, efforts, or loyalty between more than one profession, company, goal, pursuit, etc., will result in none of them being done properly. A: "His work has really started to suffer ever since he started that second job." B: "Well, what do you expect? No man can serve two masters." I thought I could write in my spare time while I paid the bills with my day job, but I find I just get too burnt out doing it. I suppose I need to choose one or the other, since no man can serve to masters.