I feel like I just went through this with my son. He just turned 16 months. Every kid is different. Some go to 1 nap by 12 or 13 months, some by 15 months or so, some even later. Unfortunately with twins, one may be ready for one nap, one may still need two!!
Anyway, when my son, who is a VERY reliable sleeper, suddenly started not napping the same and not getting his best night of sleep, I didn't know what was going on and I felt like I never knew when to put him down and he still always seemed tired!!! What we went through was a few weeks of chaos where I tried one nap, then two, then finally decided one was better. Then he only took one nap, but it wasn't very long and I had to put him to bed so early, but then a couple of weeks after that, he finally settled into a good rhythm. Now he takes one 3 hour nap in the afternoon and sleeps about 7pm to 7am.
Some kids take a few days or a week or two to make the transition and some take several weeks or more. With twins, they could be creating even more chaos because you're trying to figure both of them out (I, myself, can't stand the chaos!!). It does sound like they're making the transition and until they find their new rhythm, it may be unpredictable. (I've heard some people with twins say they just put them on the same nap schedule whether they're both ready or not, just to reduce chaos. That part's up to you.)
Hang in there! You've got a lot on your plate!
B.