Sibs bickering at bedtime - tips please!

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Ds(5) & dd(3) asked to share a room. We were happy to finally have a spare room/study and they've been sharing for a few months now. But bedtimes are SO FRUSTRATING. When they were in separate rooms we'd put them to bed and they'd go to sleep. Now we put them to bed and they either bicker and shout, or ds sends dd on missions of naughtiness which she seems unable to turn down. The ways we deal with naughtiness during the day, which usually work (time out, time apart if they're winding eachother up, sanctions & lots of praise etc for good stuff) don't seem to have an impact or don't seem appropriate (sitting on the step would mean getting out of bed, which they see as a bonus, for example). We had a pasta jar reward system for good bedtimes (which worked well when they were in separate rooms) but that also isn't working. We've tried ignoring and hoping they get bored but they have superb stamina it seems.
Any tips very gratefully received...

TBH I'd separate them again. They will need to be separated eventually anyway, and havoc at bedtime is normally a sign that it's time for them to be apart. My two shared and were loathe to separate but when bedtimes became bickering matches we put them in separate rooms.

Hi racingheart, that's been what dh has said too, I think he regrets agreeing to them sharing.
But I kinda think it can be good to share a room, I did with my db and have some lovely memories of it. Dd has fewer nightmares in with ds than when she was on her own. And the other bedroom is teeny so in some ways it seems fairer if the share the big room.
So I would like to give it every chance to work but we've run out of ideas.
It is not just that I want a spare room to set up my sewing machine