Does the Multiplatform Luminary feat allow for the selection of another Platform for the Luminary class? If so, is it possible to combine uses of both Platform's?
Would it make more sense for the Luminary class to already give access to another Platform at higher levels and then depending on what Platforms you choose you can access higher level Feats and Focused Spells that can only be accessed via two specific Platforms?
Does the Multiplatform Luminary feat allow for the selection of another Platform for the Luminary class?
No. It only gives you the focus spell.
If so, is it possible to combine uses of both Platform's?
Same as above. Additionally, "If you cast a stagecraft spell that creates a spotlight while you already have a spotlight from a stagecraft spell, the first stagecraft spell ends immediately." You can't even have both starting focus spells active at the same time.
Would it make more sense for the Luminary class to already give access to another Platform at higher levels and then depending on what Platforms you choose you can access higher level Feats and Focused Spells that can only be accessed via two specific Platforms?
I don't think that would be a great use of space. There are five platforms, which means there are (5 * 4 / 2) = 10 platform combinations ignoring order. If you had one ability for each combination, you could instead add two options for each individual platform. It also means that characters who fit a particular platform are forced to work in an off-theme platform at higher levels.
Letting the cross-platform feats give a little more does seem like a good idea, since right now the first focus spell will always conflict with something you already have.