I always found it was based from feeding. If I could feed them at the same time then it was more likely I could get them sleep at the same time. DS was a little more challenging and I did end up splitting rooms to assist with this. Are you feeding simultaneously?
I follow the eat-play-sleep pattern; the twins eat at the same times and I base daily naptimes on the previous nap/wake-up time rather than a specific time every day (though bedtime is 7 pm every night). At 5 months, my LOs could really only stand about an hour and a half of awake time, so 1 1/2 hrs after they woke up from a nap they were back down and nap times were also not "over" until at least an hour had passed (closer to an hour and a half was better). At that age they experienced lots of sleep disruptions but I tried to soothe them back to sleep rather than letting them get them up after 30 or 45 mins. Around 6 1/2 months, the routine really clicked in for them, they started settling themselves down more easily and sleeping longer, and things have been much smoother ever since. Consistency is key. Good luck!
Re: Advice on napping schedule