You can generally manage to keep 2 naps as A time increases by letting the day get a bit longer (so moving BT a bit later up to around a 13h day) and deliberately shortening one of the naps - usually this means the following A time is a bit shorter. Just before DD moved to one nap she was on something like
WU 6.30-7
Nap 10.30/11ish for 1h30 (max)
Catnap around 4.30 - 30-45 mins
BT asleep for about 7.30ish
Hope that's helpful
