The eight (owned by some group that owns 800) can undercut the two on price until they go out of business. The eight could refuse to deal with suppliers that deal with the two. The eight can exclusively license products and procedures within the area, which the two would not be allowed to sell. The eight could pour a lot of money into local advertising, and start rumors about the other two to be picked up in the papers. The eight could award itself certifications and awards that are denied to the two, because the 800 represent a lot of pressure in the industry.

Or they could play dirty tricks. I think the previous is what they would be expected to do, and if you were an investor you might be able to sue them if they didn't.