Right now, only layers can be merged together. It would be useful to be able to merge masks, too. A workaround could to convert both transparency masks to paint layers, then merge the paint layers together, then convert the paint layer to a transparency mask -- but that doesn't seem to give the right result.
Can this extend to selection? 2 selection layers merged would be the intersection of each other. Results would be similar. Also, it would be nice if you can use add or substract operation for masks and selections.
It would be good to have an option to merge it not only using AND mode (which I guess should be the default for transparency masks, as this is what happens when you have two transparency masks enabled). Selection masks are a bit more tricky since there is no way to have more than one enabled at once, but we could default to AND as well. But it would be good to have an option to merge them with OR mode in some way.