hugs Laura! obviously no experience-yet:P- but was there some kind of a perfect A time when they had that long nap? Was there anything you could see in that day that may have helped with such a nap? you said you've tried less A time but could it possibly be more A time they need? obviously both could be different in that regard too ...
Are they still together, have you tried completely separating them at sleep time so they don't wake each other at least?
Have you tried tummy sleeping? I know it's not advised overall but DS would NOT sleep on his back but slept wonderfully on his tummy, he had very good control of his neck, I'd check on him frequently etc.
Do they like soft music as a kind of wind down? will either take the dummy?
sorry it's just questions, trying to brainstorm..