We were on two capped naps from about 11mos until about 14 mos. It's what got us through and helped to keep the two naps. We capped the am nap at 50min to an hour, and capped the second nap at about an hour. He was doing an 11hr night.
We tried to go to one nap at 12mos and you could tell he wasn't ready. So, in order to stretch him until he got a bit older we did the above.
What began to happen was BT got closer to 8 or 8:15pm for a good month or two. Which was way later than his normal 7-7:30pm, but I just went with it. What made me finally go straight to one nap at 14+ mos was a combo of two things: It was taking him longer and longer to settle for the second nap, making BT continue to get pushed later, and then for about a week, he was only having 10+ hour nights. So his sleep total was dropping.
The 2-1 took a good solid month for us, maybe a bit longer but his body adjusted. For us, holding onto those two naps, even though they were capped, helped buy extra time. I would agree that keeping two for as long as you can, even when it's tempting to go to one, will make the transition smoother. When it starts getting just too wierd to have two naps, you'll know it's time to switch to one.