I don't think it would eliminate the backlog, no more than a machine with a higher capacity would anyway. Because you still have to put things in and out.
The point of two dishwashers is that everything you own fits into one or the other. As you use things, you place them into the first one, for now known as the "dirty" dishwasher. When it's full you switch it on and the second dishwasher becomes the "dirty" one. So you put things into it as you use them. When the first one is finished it becomes the "clean" dishwasher. As you need things, you take them directly from the first dishwasher, and when you've finished using them you place them into the "dirty" second one. When the second one gets full or the first one gets empty (hopefully, this happens at roughly the same rate) you switch on the second one and the first one becomes the "dirty" one again. When the second one is finished you can use it as the "clean" one and take things from here when you need them, and they keep rotating like that in a constant cycle.
It's brilliant because it means you don't use any cupboard space to store things, you don't have to deal with the putting-everything-away phase, and you don't have dirty dishes sitting on the counter waiting for the clean stuff to get put away before you can fill up the dishwasher.
n.b. I don't actually have two... just definitely see the point and would love to in the future!