can i add soil to pot to make more roots grow?

2 of my northern lights fem are in the same 5 gallon bucket but it not full all the way n i want as much room for the roots as possible if i add more soil to the bucket will it create more root growth n do you think there will be too much root stress with the 2 plants in the same bucket?

They shouldn't be in the same bucket obviously but it's too late to do anything about that.

Transplant the whole rootmass into a bigger container...or another five gallon with more soil on the bottom. Putting more on top won't give you more root space.