Not really a theory, so moved to Classic Zelda.
Termina is supposed to be an alternate dimension. Whether or not you believe this or not (I do) is up to you. As an alternate dimension, they have another set of Composer Brothers.
It's not something you can believe or not believe. It is stated as such in the Majora's Mask manual; it's an alternate dimension.
Citation from the manual if you don't believe me:
Welcome to Termina
This is a kind of parallel world which is similar to and yet different from the land of Hyrule, which was the setting for The Legend of Zelda: Ocarina of Time.
Termina is an alternate dimension of the people. This is important, as Termina is a different land from Hyrule, just the people are the same, but have different names. They also have the same characteristics (kind of).
No, sorry, it is an alternate dimension not a foreign land. The people are parallels to the people in Hyrule's dimension.
"I am Flat, a composer who served the Ikana Royal Family." He shows the same amount of dedication to the Ikana RF as the OoT version did to they Hylian RF. Unless the Ikanians are the Hylians, it's almost certain that MM Sharp and Flat are mere counterparts to those in Hyrule.
The Ikana Royal Family and Hyrule Royal Family are not the same, so there is no possibility whatsoever of them being the same brothers. Besides, their deaths are different to boot. Ganondorf killed them both in Ocarina of Time, whereas in Majora's Mask Sharp killed Flat.
I do believe that Ikana Royal Family is the equivalent to the Hyrule Royal Family, but they still are parallels and not the same, thus the two sets of Composer Brothers are not the same.
ummm Majora I don't think you understand what a parallel/alternate dimension is exactly, you see it is the same world though usually with a few twists to it, like in this case instead of a lake we have an ocean, instead of a forest there is a swamp and the main town is at the center of it all instead of a ranch.
Not necessarily. Given that the concept of alternate dimensions is theoretical and very possibly not even true, we have no way of knowing what the rules are. Many versions in fiction say its just a different world occupying the same space, not one that actually contains more versions of us. So the idea of the land being different (which would make sense from different geological activity) isn't that unlikely, even with a version that contains parallel people.