It depends on your specific contracts. If you contract with the hospitals as an independent contractor and you don't have a noncompete or some other contract clause that prohibits other work, then yes it's fine (albeit, for full time work at two hospitals you may find yourself juggling quite the schedule). If you are academic, most will contract you as "full-time" and have something in there about not having another job (in medicine, in your particular area, usually, but this can be worded however their lawyers want). If you are a part of a group you may end up contracted with multiple hospitals anyway.
Bottom line, yes it's possible, but you need to tread carefully with how your agreement with each job is set up. And to be honest, I'm not sure that you'd necessarily earn more money, if that's your goal...