Do companies mind if you work for more than one site? I think if you make decent money with one, you probably don’t need to work for two, but it’s probably best if you don’t put all of your eggs in one basket. What happens if you have a disagreement with one, or they suddenly stop paying you?
Most of the models I know work for several companies at the same time, and most companies are well aware that the models are working more than one site.