I had to chuckle a bit at this question. Daycare ratios typically allow for 1 adult to care for 2-4 infants/toddlers, so this is a very normal situation for anyone whose child is in daycare (home daycare or a center, the ratios are the same) or goes to a church nursery or anything like that. If you had two children it would be the same situation; what would you do when one of them needed you? You would help that child, and the other one would wait. You would be quick, maybe you would hold both at the same time, but for most people (either because their children are in daycare or they have more than 1 child), not being able to devote every moment to just one child is normal, and the children do just fine. :)
Now, I have never used childcare at a gym, and there are other concerns I would have. How old are the childcare providers? What experience do they have with infants in particular? Is it the same people or is it always changing? Will they change and feed your child if necessary, etc. But the simple fact that one person may be caring for more than 1 child is going to be the case whether you take her to the gym or mom's day out childcare place, so I would just find a place where you feel comfortable taking her, and do that. Personally, all other things being equal, I'd prefer for my child to be at the gym with me because that way I could attend to her easily if there were an issue while I were working out.