I'm not the best on sleeping issues but I am a childminder and have an 18 month old daughter and care for a 9 month old little boy. My daughter was used to napping quietly before I cared for the little boy but of course, if he cried, he would wake her. I persevered and soothed her to sleep without picking her up. Basically, I was adamant that she sould sooth herself to sleep even if there was noise around and encouraged her to nap with television, radio, play, chatting, any noise in the background. She now sleeps throught just about anything (except whatever wakes her around 12midnight)
The twins are still young and have lots to learn so I'm sure that this is only short term and they will find their routine soon. Good luck.