Don't think thats quite right. MBNL is a joint venture between T-Mobile UK and H3G-UK. The T-Mobile part has now been assimilated into EverythingEverywhere. MBNL operates transmission sites for both T-Mobile and 3. However, both networks retain sperate back haul service and their own frequency, even at these shared sites. This is a 'site share' scheme, there is no roaming between 3 and T-Mobile's 3G networks. A certain level of 3 signal does not guarantee T-Mobile coverage, and vice versa. Each network makes individual decisions on mast power, etc based on their network demographics. Also bare in mind not all MBNL sites operate both H3G and T-Mobile masts, the majority do, but not all.
The plan is for some previous Orange UK locations to join MBNL - presumably in areas where the combined network needs the capacity, I think the eventual plan is to have one, single, EverythingEverywhere network. When I was at Orange the tagline was "Orange, inspired by EverythingEverywhere"