Yeah I'm with Kel, if I can't resettle DD2 within 5 minutes, I get her up and put her in the sling to continue her sleep while still doing stuff with the other two.

No use getting stressed out trying to get a baby to sleep and worrying about your other one - it doesn't help anyone. Bubs can feel your stress and won't go to sleep just cos you want them to - unfortunately!!!! LOL