ALso with a 3yr old and a 6mth old. Do as follows---
6pm- both bath. Then leave 3yr in bath playing and get 6mth old out, dress him, sit him up with something to chew on where he is involved in the activity,
get 3yr old out of bath, in pjs, then, get both LOs' milk downstairs, leave 3yr old with milk and to play for 10mins, take 6mth old into bedroom (that they share), give milk, finish that, dummy, asleep, close the door. Now about 6.45pm.
So, then to storytime for 3yr old, in our bed. 3 stories, then he lies down to sleep, in our bed.... or possibly transfers to his bed, but by this time 6mth old pretty quiet, so no issue. Have finally taught 3yr old bedroom is quiet room, so he tiptoes in there and whispers, lies down, generally just a few stalling techniques from him and minor grumbles.....
Of course, I say all this as if it's been a piece of piss getting there. A couple of months ago I was re-considering the whole bedroom sharing thing, cos all they did was keep eachother awake, and 3yr old's constant nonsense about getting into bed was affecting 6mth old's bedtime routine... but FINALLY, recently, it has slotted into place and works really well!! (!)