Sophie honey you're doing your best, its hard enough getting one baby to sleep let alone two and its just that now you'd probably like them to be doing it a little differently.
They are now in a routine or habit of sleeping this way and all you have to do is make them a new routine. The bad thing is they will probably protest like mad at having to learn how to fall asleep the new way, i.e. on their own. However, when they start doing it happily you will give yourself a huge pat on the back. I've been here myself (with dd, she loved falling asleep on my chest to my singing, not even good singing ) and once I broke the pattern I wondered why I didn't do it earlier.
Loads of books on this but simply they need to learn a new bedtime routine, something like - bath, bedtime bottle, story next to cot in dim room, cuddle time, teddy maybe to take to bed, lie down and tuck in (I put on a musical wind up at this point), kisses and leave the room.
They will probably cry like mad at this point because its not what normally happens. You need to wait a bit before going back in saying lovely things like how much you love them and its time for bed now and night night, kiss again and leave again. Leave it longer and longer before going in each time and if they sound like they are quieting down, leave them. You need to keep repeating this until they fall asleep.
When I broke the habit with my dd I was lucky enough to do it when she was young and in a cot. It would be so much harder trying to keep an unhappy child in bed once they are in a big bed. I was also lucky that I had only one night of unhappy crying (about an hour in and out, and no picking up or cuddling, just soft words). I know some kids take much longer.
It wont be easy and will feel like its not worth it BUT IT IS. You'll need to decide whether to teach them both at the same time or one by one, and that will depend on your own situ, i.e. if they have separate rooms etc. Personally I'd go for it all at once, in for a penny in for a pound.
My DTs go to bed really well (after my mistakes with dd), alone and in the dark. They each have a special teddy for bed and have their musical wind up on (just once). Once the door is shut I sometimes hear them chatting to themselves and its a lovely feeling. I can then get on with the serious business of opening the wine .
No matter how much we love them, its generally a relief when they are sleeping happily and we can breathe easy for a bit, best of luck honey and I hope they respond quickly. ITS WORTH IT and ITS NOT CRUEL.