Well, it all depends. Do they live near to each other so you can see both? And if one asks you to stay, kindly say "no" because you have to go see the other. But, like I said, it depends. If your mum is seriously ill, then I'd go see her and just be fine with calling my bf. But then again, if your bf really wants to see you, atleast go visit him once or twice. I'm sure if you told your mum or bf that you wanted to go see the other that they'd understand. Atleast you aren't leaving them for a really long time.