My DS is 8 months and only really started with dadadadada about a month ago, but has now in the last couple of weeks started baba and mama. It just depends on the child doesn't it? Some of his wee friends the same age are crawling and he isn't. I'm not worried. You're wee ones will babble soon!
No early development stages are set in stone. Children usually start sequencing sounds (baba, mama, dada etc) at AROUND 6 months of age, but there isn't a hard and fast rule for it. So don't worry! Chat to them lots during daily routines, they love the sound of their parents' voices. Holding them on your lap so they can see your face and copying the sounds that you make will encourage them to 'chat' more. Try extending what they say e.g. baby says 'ooh', you say 'boo-boo'. Have fun!
DD1 started quite early - at about 5 month. though didn't mean a lot. she is now 3.4 and has a severe speech and language disorder. DD2 is 6.5 month and started with dadadada at 6 month, also a lot of ayayayaya. no mamamama yet.