M.P.
I suggest that you just have to do it. Put them together in the room and give them time to get used to it. You don't say in what way it was a failure. Did one wake the other one up? Or? Did they not go to sleep because the other one was awake? Or?
One solution might be to do as Momma L. suggests. Get them to sleep in separate rooms and then put them together. Or let the littlest one sleep in the living or dining room.
There is a difference between 6 months and a year. Is she now sleeping thru the night?
I think you just have to be miserable for a few nights/a couple of weeks until they get used to it. Treat it as sleep training. Go in after 5 minutes or so and calmly pat each on the back until they calm down. Gradually lengthen the time before going back in. The idea is to teach them that you will take care of them while teaching them how to calm themselves.