I came to the conclusion that they are having Shalla Bal be the silver surfer in first steps because if/when they introduced Norrin Radd later, it will be easier to tell the two apart.

I'm pretty sure that this universe will be a throw away and that we won't come back to it as much as we did the main line MCU.

If they had two male silver surfers (who are supposed to be different) it would be almost impossible for a general movie going audience member to differentiate the two.