I sincerely hope you have been successful at supplementing these two little ones. I hope you will check back in here with us and let us know how things have gone.
In (late) answer to your question about either regular or soy baby formula, the answer is no, not for any prolonged period of time and not very much. You will want to either find goat's milk or a pre-mixed kitten milk replacement formula. There are many brands of milk replacement, many people like and use KMR. You can also ask your vet what he recommends and to help you determine how much you need to feed as well as how often based on weight.
Are they using the litter box by themselves yet?
To help your Momma-kitty, make sure she is eating a good high-quality ~kitten~ food in both canned and dry varieties. Allow her to eat as much and as often as she wants. You might also consider offering her a small treat of plain yogurt, warmed goat's milk or milk replacement formula. This will help her to regain her strength and begin to build her resources back up after nursing six kittens for a month.