Hi Heather!
I just was trying to figure this one out too! In one of my sleeping books I think it says something like: 80% of children are still taking 2 naps at 12 (or 15?) months and 20% are still taking them at 18 months. My 18 month old just switched to one but it starts around 10:30am and goes to about 2 or 2:30pm. I can start bumping back 15 minutes or so a day to transition her to a true afternoon nap I have read. Some great books that will tell you all about it are:
Healthy Sleep Habits Happy Child
Baby Wise (there are a number in this series)
The Secrets of the Baby Whisperer
The Secrets of the Baby Whisperer for Toddlers
All in all, don't try to switch them too early. I have heard that big day cares switch kids from one nap to two immediately at one year. From what it sounds like there is no real transitioning there. That sounds pretty unfair to the children.
Read up in some of those books and it will help. Just pay attention to your babies and their moods, etc. and you will be able to tell if they need more sleep or less. It could even vary day to day.
Good luck!
Best,
A.