Here's a few ideas:
1. a) If you have the money, give them both a shot of baby boost. b) Then put them together to try every 20 minutes or so until they produce a baby (usually works for me).

2. If they're arguing I keep putting them together until they go try and then follow b above.

3. I also remember someone saying they put perfume on their peeps and then put them together.

Make a back-up of your game just in case and good luck.