We have two bostons, and from our experience I think you'll probably want to have separate crates - especially if one of them already things of the current on you have as their own. Our Bostons are pretty defensive of their little homes inside our home
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To answer the original post...
We first bought our youngest, Chloe, as a pup and soon afterwards struggled with the idea of getting a 2nd. We went back and forth for a couple of months trying to decide. Then we heard from a friend about Duke. He was owned by an older man, who had passed away, and Duke was subsequently passed on to a couple of relatives. They apparently never really were "dog people" and they pretty quickly got fed up with Duke's antics (marking in the house, storm anxiety, etc.). So anyway, we heard that they were trying to find a new owner for Duke and that they'd pretty much given up and were planning to just drop him off at the pound.
So we took Chloe over to meet Duke. They hit it off pretty well, so we "rescued" Duke from his owners.
We, Chloe included, couldn't be happier. Our two bostons seem to be great friends. They play with each other several times a day, know just how to push each other's buttons, and they clean each other (tongue baths) regularly.
As esther said above, it's going to be very dependent upon the personalities of the dogs & people involved (and in your case, the cats too!). In our case, it turned out to be a great situation for all of us!