Noticed this never got answered, did some research, answer is yes.
Wanted to confirm you can have multiple souls active at the same time, and the answer given by bossman Caspian is yes. If you pay for multiple souls, and put sparks of life in them all, you can have multiple characters. Just kinda burning the candle at both ends because they will both be utilizing their lifespan simultaneously. Source 1!
If you want to also play your child you would do it in the same way you would give your child's code to another player. Since there are 3 different ways to handle the child's soul, Lock it, Share it, and Free post it, you would essentially be sharing it, but with yourself. Source 2!