There does not seem to be a clear line between the two license holders. As an example;
IPB and TPF can produce MM playfields. Both have said they will. IPB have released theirs.
The "who owns what" lists are shrouded in mystery. Either way - Both companies produce what they think they have rights to. The only governing factor in the mix is WMS Industries - Who have to approve the right to produce the licensed part(s). I think they simply request permission from WMS Industries to produce.

So really its whoever out of the two gets into WMS first or whoever has the financial resources to back their money making idea that gets granted the rights to do this............