Oh it's not easy, I didn't mean to belittle the challenge ahead.
Given that the girls are already used to boundaries, they just need extending into bedtime and holding firm, as it were. I understand this is easier said that done. It will no doubt involve lots of tears and tantrums and hard work at first.
Are they in different rooms?
If there is argument about which parent does which then I would be a team effort. Could your 6 year old entertain herself for half an hour while you and DH together do the 3 year olds bedtime? Divide and conquer the girls.
Mummy, Daddy, DD2 all together for teeth, PJs, Story, Bed. Kiss goodnight and explain that you are now going to put DD1 to bed so will come back to DD2 afterwards. Then leave.
Leaving (ie not staying with her at bedtime) will take some work and will be difficult at first. So explain what will be happening way before bedtime so both know. You will put DD1 to bed, Mummy and Daddy giving her sole attention. Then will leave her while they deal with DD2. But will come back.
The Mummy, Daddy, DD1 all together for teeth, PJs, story and bed. Kiss goodnight and leave.
Go back to check on DD2 but don't stay. Agree to hang around upstairs.
Both girls may respond well to sticker charts. DD1 gets a sticker if she stays in bed while you do DD2s bedtime. DD2 gets a sticker if she plays quietly while you do DD1s bedtime. Then both girls get another sticker if they stay in bed afterwards. Then a special gift or day out if they get, say, 10 stickers each over a week.
I realise I am blasé about 'just leave her to go to sleep' and she'll scream and scream and this will be horrendous. You will need to agree a plan of action in dealing with this with DH. Be it safety gate on the door and just ignore while you both put DD2 to bed. Or you agree one of you will keep returning DD2 to bed while the other does DD1s bedtime.