Who's easier to get to sleep? Get them to sleep first then deal with the second. If I'm getting DS to sleep I will let DD watch peppa pig and playschool on my iPad then I deal with her...Ok so sometimes with DD I get desperate and if she's mucking up too much (she would literally stay awake all night if I don't be stern sometimes!) I tell her she can sleep outside and if she still mucks up I will pick her up and put her outside, that will literally only last a second as she hates it. I give her a hug then say ok if you don't want to sleep outside then you have to sleep in bed. Bad Mummy! But better than letting her CIO though right?
DS is easy and will just fall asleep in my arms in the dark after a little while, he is a VERY different kid to his sister which was settling in the fact that I knew it wasn't normal with DD.
good luck hope you find something that works.