Just found in Tracy's last book, some tips for the transition of 2 naps to 1 ...
By making the morning nap progressively later, you can eventually make it disappear! The timetable below, assumes that your child is at least 12 months old and normally naps at 09.30. The times may vary in your case, but the principles of gradual transition remain the same:
Days 1 to 3 - Put her down 15/30 mins later for her morning nap - 09.45/10.00.
Days 4 to 6 - If possible, put her down 30 mins later, so that she goes down at 10.30. Give her a snack at around 09.00/09.30, she should sleep for 2/2.5 hours and have lunch around 13.00.
Days 7 to however long it takes - Every 3 days, make the nap later and alter, she might have a morning snack at around 10.00 or 10.30, go down at 11.30, and wake for her lunch at around 2. You might have a few rough afternoons.
The goal - Eventually she'll be able to stay up until noon, have her lunch, play for a while and then take one long afternoon nap. She may have days when she can't make it without a short morning nap, go with the flow, but never let her sleep for more than an hour in such a case.
I know that you've managed to cut out the morning nap, but maybe these tips will be helpful in extending the afternoon nap.
Hope this helps, please keep us posted with your progress xx