Savannah is a great name, although it is becoming more and more common (I named my kids common names, too). How about Savannah Marie? I would worry more about the first name sounding good with your last name rather than the first and middle sounding good together, since that is what people will know them as. I also wouldn't worry about the names sounding too much alike. They won't always be together.
I am going back and forth about giving the kids the same middle name. They are going to be separate adults some day, but they are still twins and it is awful cute. Are there any other good family names you could use for middle names. Hov about Ava instead of Eva?
I am sure once the girls are born, the names you pick will fit them perfectly.
Congrats, and good lucK!
Denae
After checking Robin's link, my girls were #1 and #2 the years they were born. I guess I did this on purpose after growing up with such an unusual name - which I really like now.