i don't do it - but i know for some families co-sleeping works for them. it scares me too much to do it! i remember once i fell asleep with my daughter in bed with me after nursing her (she was about 6 weeks old and her older brother was just 14 1/2 months old. needless to say i was exhausted!!). my husband came to bed and put her in the pack n play next to the bed. i woke up and freaked out because i didn't remember him taking her out of my arms. i now can't sleep with them in bed! even with my now 2 1/2 year old who very rarely comes in there - and he's old enough that i dont need to really worry about him.
it's a tough call at 6 months. some babies aren't ready to sleep thru the night yet. i think my pedi said that at 7 months babies are able to regulate their blood sugar and should be able to sleep thru the night. Is your youngest teething? or reaching a milestone? things like this can cause regular sleep patterns to be pushed to the side. if so maybe you could put her in your room or a guest room (if you have it) until she's back to sleeping regularly.Is she sleeping regularly? Maybe you could try putting on some white noise music. that may help drown out some of the fussyness. i don't know if its a good thing or not to remove your youngest from the room. i would worry then that your older daughter won't get used to the noises of her sister. but it's important that she gets her sleep!!!
Sorry i'm not of much help! Good luck!