Hey there, welcome to the forums. And wow you're from Japan? Lovely place.
First off 7g can house no other fish friends. Maybe some shrimps and a snail, but I would say no to other fishy friends.
If you choose not to add shrimps/snails, dividing it could let you house two Bettas. If you're male Betta is comfy in his 3.5g I wouldn't move him, get another female, I'm not sure if a male and female sharing a tank would be a great idea. I have no knowledge if they can share a divided tank peacefully or not, the female might get eggy and the male might blow bubble nests like crazy.
If you would like some other critters moving about, a snail and some shrimps are a good idea. :) Ghost shrimps are inexpensive, if your Betta doesn't eat them as snacks get more pretty ones like Red Cherry Shrimps, etc. As for snails, I would get a Nerite snail, they can't reproduce in freshwater only brackish water, and I believe they stay smaller than an Apple snail.