Is either dog underweight or overweight? If so, I would listen to the vet. If leaving food out is working, I wouldn't worry about it. We free-fed our first yorkie for over 16 years and it was never a problem (except when we realized that a crafty mouse was raiding the bowl and night and storing some in a bottle under the kitchen sink
).
We have found that for our dogs, food is less important if they don't have to worry about it.
but if you have a chowhound or a dog whose diet must be measured/controlled, I would still feed at least twice a day. These guys are too small to go 24 hours without food.