The "tuning" part is just a small part of the software process (most of that process occurs in hardware). The tuning function was designed with multiple tuners in mind.
There is an entire architecture built within windows to support defining your region, your tv signal provider, their lineup, the channels within that lineup and the programs that are on within that lineup. Then there are the processes that download the guide data, manage recordings, recording conflicts, searches, etc.. These processes all expect to only have to handle a single TV signal source.
My understanding (again based on comments from a few years back, prior to Vista) is that rebuilding MCE to support multiple tv signal providers/sources means rearchitecting a large number of areas of the TV functions.