You're just asking for your money back, so I don't believe there would be a karmic backlash for that. The karmic backlash would come if you did a spell to have them get into an accident or have them fight all the time. If you did something bad to them, that's when karma would come into play.
Even if you're not a witch, YOU should do this spell yourself:
Since two separate people borrowed money from you, use two separate pieces of paper and write the amount owed to you from each person. Under the amount, write "pay me back" on both pieces of paper. Fold each piece of paper three times. Find one of your favorite pictures of yourself and place the folded pieces of paper on both sides of your picture. Use magnets to hold everything together, to attract the money to yourself.