As for the color combination, it would be something like this. I was looking for something blue-purple that would do well in shade and continue blooming with the begonia during the summer. Lobelia just poops out.

Depends on what begonia. Some begonias would take over the pot while others would probably be shaded out by the streptocarpella.

I had a pot of two begonias (Lois Burks and Withlacoochee) with a streptocarpus in it and it worked the first year. Overwintering was a big mistake as they all took a hit in the same pot. I would've been much better off if I had dug them all up and put in a much smaller pot for winter.

Here is a photo when they all looked great the first summer. Lois is the one with tiny spotted leaves and reddish blooms. Withlacoochee is on the left side with white blooms. The big silver leaves is of Judy Cook in an adjacent pot.

That's looks like a fair competition between them all. If one started taking over the other and I didn't like it, I'd probably just prune a little to even things out.

As for overwintering, I'd take the whole pot apart or cut out the streptocarpella, because those are easy to propagate or cheap enough to buy again. ;)

I just posted this and saw your second post come in. So you will need a much larger pot to combine the two you showed. I think it will work as long as the soil does not become soaking wet which tends to rot the tubers. I had tuberous begonia with New Guinea impatiens before in a coir lined basket - they did pretty good even though the Atlanta area is not great for tuberous begonias.