Try reading "Healthy Sleep Habits, Happy Child" for some professional advice on sleep habits of infants and children. This book has helped me a lot in getting through the various sleep stages of my daughter. Also, I think some kids are just 'programmed' to get up at a certain time and there's not a whole lot you can do. My daughter goes to be around 7:30pm at age 2 and usually gets up between 7-7:30am. I have tried putting her to bed earlier AND keeping her up later, and it doesn't make a difference; she usually gets up around the same time. The only real difference is that she's crankier in the morning if I keep her up too late.
My nephew, who will be 3 in February, ALWAYS gets up at 5:30am, even if he's been up until 9pm. My sister has tried EVERYTHING to keep him asleep longer, but he's been a 5:30am guy since about birth.
Now, every kid is different, and every family has a different schedule, but trying to get kids to sleep later just doesn't work for my family. However, your daughters are only 8 months old, so this could be a phase that you just have to suffer through. Have you tried going in their room(s) in the morning, giving them some books/toys and then YOU try to go back to bed? My daughter got up too early this morning, so I am feeling your pain today!
GOOD LUCK!!!