At 20 mos, one good nap is more then enough. Since you are even seeing a pattern where they just aren't tired at 10, just give them lunch say around 11 and lay them down at noon. Sleeping too close to dinnertime and bedtime will eventually backfire on you. Both of mine went down to one nap by 18 mos and I did it from noon until 2:30. That way by the time they went to bed they were tired again.
Do not let them stay up until they ask, just get them on a schedule and they will adjust accordingly.
The older they get if you let them sleep too long in the late afternoon you will regret it, they will be bouncing off the walls at 10pm! Just put them down after a good lunch and let them sleep no later then 3:00. It may take a week or two but they will adjust just fine. Move their bedtimes up too, at 20 mos they shouldn't be up until 9pm, you need to set the schedule. If they are showing signs of being cranky, put them to bed by 7:30. My four year old still goes to bed at 7:30 and sleeps until 7:00 am.
That way you and your hubby can have alone and downtime, which is crucial! :) Good luck.