I was going to say "Wow! That's great that your child has had two solid, predictable naps/day up until this point!" My son was never good at taking a nap and we struggled with that until he was a toddler. He never did take two naps/day. We were so thankful if he managed to get one in! We found that just getting him to bed earlier (when he was that age we put him to bed at 7:30 or 8:00 pm) meant he slept longer. Now, my son is nearly 4-years-old and takes a 2-3-hour nap every afternoon after lunch. He is super high-energy and he's growing really fast (4 cm in 3 months this summer) so when I see him at lunchtime after school often he'll say, "I'm so tired!" He still goes to bed between 8-9 pm. So, I feel thankful that he's held on to that afternoon rest--probably out of habit.
I say that if your baby isn't showing signs of burn out (rubbing eyes, whining, fussing) and is only doing one nap a day--just capitilize on that one nap--try to aim to have it earlier in the day (after lunch) and then start the nighttime routine (bath, story etc.) earlier and see if that increases the sleep time. They've found with babies that the earlier they go to bed, often the better and longer they sleep. But...like Koan pointed out...you might just have a kid who is a no-nap kid (I certainly was when I was little). My mom would just tell me to lay down even if I didn't sleep so I had that sort of discipline/structure in my day--for my mom it was more about her getting a break than me sleeping anyway so as long as I played quietly on my bed she was okay with that.
