It depends on your own comfort levels. I myself started doing daycare when my 1st was 9 months old. The child I took in was 12 months old. So I had a 9 month old and a 12 month old, the same scenario you are asking about. Except mine was the younger. Its essentially like having twins. The thing you need to think about is that the 9 month old will need 99% of your attention because they are so young, so you may be taking attention away from your own 12 month old who is also still a baby, but should be a little more independent. My rule is that if my own child is still a baby, (less than 15 months old) I would want to give most attention to him/her and have all daycare children older than my own. Don't put yourself in a situation where you might feel guilty. Also, don't feel that because its your first interested parents that you need to take them. More will come a long, wait it out until you find right family. I hope this helps,