Hi Sarah,
There are a few systems our there that can technically handle multiple sites, as evidenced here above, it comes down to what your priorities are, what you want the software to do for you and also how complex your operation is.
Given that you have mentioned Timely and MindBody this gives me an indication of the sort of system you are looking at and also the sort of budget you have in mind. In this case I would HIGHLY recommend you take a look at Baxus (also mentioned by Cathy above). I have been working with this software (and it's predecessor) for a while now and in my personal opinion, in this level of the marketplace it is the hands down winner. Not only is the software great, but the support is fantastic too . . . anyone currently using ANY software system will understand how important this is.
Best of luck . . .
-Adam