A: To refresh, Grafting says “Two Keepers can now be joined together to form a single two-digit number for the purposes of Victory Rule calculations.
Combine Digits says essentially the same thing: “You may join two of your Keepers together to form a single two-digit number when using Math Rules to compute the Goal value.”
A: You may only create ONE two-digit number to use in your calculations. Of course, you can rearrange your cards to experiment to see how they can best be used.