Hi there,
Two things. First, I definitely agree that the girls sleeping together may be interrupting their sleep. I've seen that over and over with my kids, that they don't sleep as well with anyone as they do alone.
Second, a fifteen month old's schedule really seems to be something they create for themselves. You can keep kids up later, give them naps or stop giving them naps. put them to bed earlier, and still they may just wake up too early, because that's when their internal clock is set. Sleep deprivation tends to make sleep more difficult rather than easier. They sleep less when overtired.
As for self soothing, I truly believe the idea of it is a myth. Babies and toddlers aren't programmed to stop needing mommy.
For help with sleep issues you can look at www.drsears.com. He has a lot of useful facts and tips.
Good luck!