Aww---they sound so adorable!
As far as the sitting, less support is sometimes better - forces them to 'do it themselves'. My twins had early intervention services the first year and at that stage our therapist encouraged us to give them just a little support at the waist when they were sitting. We also propped up toys at eye level so they had something to focus on that made them keep their head and shoulders up. When they were drawn to toys that were on the floor only, it made them slouch down to look at them...and then they'd fall over (which was always cute and made us laugh!). Also, you can help by positioning them so that they use their arms to support themselves (in front and/or to the sides). It helps with their balance and soon they'll get the hang of 'catching' themselves as they lean from side to side. Sounds like they are coming along. A few more weeks and I bet they'll be pros at it!
we never got them a bumpo chair. they have their bouncy seats. lol which are a god send. i love those things. they try to sit up. looks like they do little baby ab crunches but they never really make it to far. and if you sit them up they either fall to the side or throw themselves backwards.
i just can't believe they're gonna be 8 months old in 1 1/2 weeks!!! where does the time go?
So cute! The time does really fly by. That is great that they are saying mama and baba. Our 17 month old really only says all B words (bye bye, ba ba, babe, etc.) and mama and da (for dad). We are wondering when she will start to use more words because she understands a lot.
I think they will just sit up on their own and there probably isn't much you can do to accelerate it. Their stomach muscles have to be strong enough. The bumbo chair is good for sitting them up but there is a lot of support. They may just be those babies that go right from the floor to crawling : )