Multiple Semiclasses?

Currently there are not many semiclasses...
But for when we have more, it is likely that many people will attempt to try more than one...
I'm wondering if you can have two semi-classes at the same time.
If you cannot use more than one at once, can you switch between them back and forth by going between semi-class trainers?
I'll post suggestions on them as well.

At the moment, you can take only one semi-class (after reaching at least 10th level as a character).

I have several more semi-classes in the works, with ideas for more after that. I do plan on eventually enabling a character to take on a second semi-class after reaching 20th level or so and after taking all 5 levels in the first one, but that is quite a way into the future.