I had a really hard time getting my babies onto any sort of a schedule (they never seemed to nap longer than 30 or 45 minutes) in the beginning. When they were about 7 months old, I started working really hard at it, and I think they were ready. So, you may just have to wait a couple more months. Aside from just being older and more regulated, at seven months my babies could support themselves enough so that I could lie down and nurse them both at the same time. This is HUGELY helpful. It is still how I get my guys to take a nap when we're home. I lie in between them and they both nurse to sleep and then I get up and they stay there sleeping (in our family bed).
Another idea would be to take the babies for a walk in a twin stroller (assuming they like strollers--mine didn't until 6 months), and have them both sleep in there. I still do this somedays when I can't get a nap to happen otherwise, or when I just want a really easy to-nap period.
Now that my babies are friends, it can be really hard to get them to take a nap at the same time because just when one is almost asleep, the other will sit up and start giggling. And then it's all over. So, when I'm not home alone with them, I put them down for a nap one after the other.
I also now really try to enjoy the times when one babe is sleeping and the other is awake. My guys have such different personalities when their brother isn't around! It's amazing!
I hope some of this is helpful. It will definitely get easier before too long.