We are in the middle of 2-1, so no easy answer from here. DS (16mo now) can stand even 5,5h A time, but he does not nap consistently for 2h and his night is never longer than 11h, so in 1 nap days he reaches BT stretching too much his last A time.
Re BT, for us most important thing is when the pm nap ends. If it ends around 3 or 3:30 DS is ready to go to bed at 8/8:30pm (we do a 7am WU and 8pm BT and anything before 7 is EW) and we do not have EWs. More or less same approach for am -> pm nap. He needs at least 3h from waking up from am nap to be able to do a pm nap. Even if the nap is as short as 10min. So I found myself furously APOPing an early am nap and when it does not work, day is rubbish.
Anyway some wise ladies in other threads told me that moving to 1 nap is ok when babies can stand at least a 5,5 A time, otherwise the night becomes too long and this can backfire quickly.