I think there's a bit of confusion here, again as people aren't getting the jist of franchising.
In terms of the wider deregulated bus market, there are concerns about monopolistic positions caused by acquisitions, and that's when the Competition and Markets Authority get involved. HOWEVER, THIS ISN'T THE CASE HERE! (sorry for raising my voice)
Instead, TfGM are the ones who are solely responsible for the management and planning of the Greater Manchester bus network, running a set of tenders in order to get the Most Economically Advantageous Tender (MEAT). That means the best value (for both quality and price) according to the specification of the tender that reflects what TfGM want.
I don't think they have a cap on the number of separate packages that any business can win. Not certain that procurement rules allow that but I may be wrong. However, there is clearly no issue in awarding large neighbouring packages to the same supplier - see Go Ahead (Bolton and Wigan), or Stagecoach (Oldham and Middleton, and arguably Queens Road).
Whether there is a longer term view that allowing one business to gain much or all of the packages might have ramifications. As
@mangad said, there is increased risk with a business failure though there are certain provisions regarding Parental Company Guarantees and Performance Bonds to address this. Also, it might provide an ongoing advantage to a single organisation if they have economies of scale that they can leverage. It will be interesting to see who gets the T3 awards but might not simply be the same faces again.