Shawn, I've seen this approach tried a couple of times locally. Unfortunately, what I witnessed in both cases was problems cropping up fairly early in the relationships. One of these actually spent big bucks putting in a mall (for the lack of better description) with several store fronts on a prime piece of real estate. The store fronts ranged from an Audio Store, Tire Store (too small from the very beginning), Speed Shop, Body Shop as well as a first rate Detail Facility. This looked like an ideal set up from what I could see.
One of my friends was tied into the Speed Shop and a very decent engine builder. This was the vantage point where I started to see things head South. Employees started jumping ship from one store front to the next, there became duplication of efforts between the businesses etc. The tire store started to conduct most of its business through Tire Rack. Not that this is bad but very little inventory on hand, so they wanted to do suspension work (Speed Shop turf). When people want tires they pretty much want to get them the day they go shopping. Wheel customers are more inclined to be patient. Mind you these were all independent business that were renting space from the guy who owned the Speed Shop, my friends boss. He kind of worked like the Mall Management.
My buddy thought that the monthly fees were very fair and problems arose elsewhere. The real issues came when one of the businesses wasn't living up to its commitments to its customers and this reputation started to hurt the other businesses. Maybe the problem was the proximity of these shops being so close together. This "Mall" went under in about a year and a half. The Audio store actually still does business down the street. Maaco ended up picking up a couple of the other shops.
I think most car guys would like to see a one stop, we'll do it better than anyone type shop. This would be true either as a customer or as a proprietor. I mean it really sounds great.
Let me ask you this? When a warranty issue crops on an engine rebuild for instance, who eats it? Who does the customer direct their concerns towards? Do they just bring it back to you or do they take it to the facility that actually worked on it? How does the warranty expense affect your cut? These questions might seem like they're cutting to the quick but this is where I saw problems. I also saw what I thought was a pretty good friendship turn into a real feud.
I know this isn't the most positive post regarding this type of service. I actually hope this flies for you.