Oh Andrea… Always playing devil's advocate...
I prefer to have one food bowl for my dogs. It's just a preference. My longtime dog is used to 1 bowl no matter how many dogs. And she free feeds. We went to 2 bowls when we added our rescue, Miles, and it actually seemed to promote more aggression in our home. And that's just for our situation. Lexi seemed to go, "Wait, where's my food?!" when it wasn't on the floor all the time. And Miles was just hungry. So trying to put food down was nearly impossible without them getting into a spat saying, "Get in line buddy, I've got first dibbs on these bowls!!" It was always a very frantic and overly excited time when things could go wrong- they drew blood twice the first week.
So for us, once the two dogs got used to each other, it's worked well to have just one larger bowl. They can both free feed without over-doing it. And they take their turns which I personally feel helps with the rank structure here in the house. Everyone is very respectful of everyone else. Plus they were making a HUGE mess when we had 2 bowls. They'd run and drop food all over the carpet and also all in between the bowls as they'd check each others' dishes out to make sure the other didn't have something better. With one bowl they aren't doing that. It just works for us.
We pay attention to who is eating and when, but you're right, we don't know how much each dog is eating. But we do know that both dogs are eating and that's what we need right now. If one's weight started to drop off, or if we noticed one wasn't going to the dish as much, we could do something. But I personally don't think the putting the food dish on the floor is the only way to have control in the house. Besides, they still watch us put the food in the bowl. They know that we are the ones that give them food.
Our dogs are our kids. And we pay attention to them unlike pets, but rather like our sweet babies. We know them and their habits (getting to know Miles still obviously) and are very aware with something small is off. Having one bowl just works for us. Not everyone will have the same situation. You gotta do what works for you.