Yes I can see that and understand after seeing a friend's school go under because it had it's main financier leave and it basically collapsed under it's own weight. So yes there is that; one bb/instructor from one art partnering with another bb/instructor from another art and both of them doing on/off nights at the school to help keep it going for the both of them... even three instructors could do this.
Which is why I said when I do go to one of these (because at the moment where I live the choices are few and far between) I plan on bringing a proverbial microscope and the ole' KY and check them out... with the folks on MT for assistance when I get the names of THEIR instuctors and post the question: Has anyone ever heard of "this guy?"
Re the first part: Theres nothing wrong with offering more than one art, if you do it right. There's a school in a very upscale part of my metro area. The owner is a very established, high ranking expert who has been around a long time. Somehow he got together a lot of dough and built a large attractive complex which houses a "martial arts center". It contains three separate glassed in studios which can be viewed from a spacious central lobby. He contracts with top notch, well established people in the region to teach a variety of traditional arts under one roof. In addition to his particular ryu, ongoing courses are offered in Judo, Iaijutsu, Chinese Wing Tsun, and other arts. Each of the different instructors runs their own quality program and the owner takes a percentage. And, if in fifteen or twenty years, the owner retires... he's got most of his money in a very nice piece of real estate. I tell ya, it's so nice that when I'm out that way, I like to drift into the lobby and just watch the training. Of course, I'm friends with one of the guys that teaches there. But even so, it's just a very comfortable place to visit. Now, if I were ever to open a commercial operation, that's how I'd like to do it.
Now about your second statement. I agree that doing your research is important. But it is also true that there are some great people out there who don't have much of a traceable history. I study escrima with a guy who is terrific, dedicated, and has been around a long time. But, he's got his own system going, and never bothered with getting any kind of certification from any big name people. But if you know the art, you know he's for real. I think if a person visits a class and really watches the quality of the instruction, they will get a truer impression than by looking at certificates on the wall. Asking around on a forum like this isn't a bad idea either.