Is there any reasons cats can't eat canned dog food and vice versa on occasion, when there's a temporary shortage of one or the other?

They do and will but it is not good for them.
Like feeding your kid junk food to fill them up instead of meals.

From time to time is okay, but it's discouraged. Any sudden change in diet can cause GI upset. Just buy more before you get to the point where you have absolutely none until you buy more, because then you run the risk of it not being in stock..

For long term - NO. Cats and dogs require different things in their diet, so food manufacturers ensure they have just that. For example, a cat requires taurine in their diet, something most dog foods do not have.

I'm going to give a different spin on this. The vast majority of pet foods aren't really appropriate to them to begin with. In other words, feeding them your average (or even so-called premium food) is like feeding them junk food anyway. Most manufacturers don't actually ensure they get what they need. They ensure they do what they can get away with and the requirements on that are not strict enough. I'm glad you said canned, though, cause that's going to be better than dry.

Cats and dogs both are carnivores and most foods have a lot of other junk in them. The difference is that cats are obligate carnivores, meaning they must have meat. Dogs can more easily get away with eating less optimal diets, but they still should really have a carnivore diet. (As one web site puts it, it's the difference between surviving and thriving.)

Because the taurine is an important issue, it's easier to get away with feeding a dog cat food than it is to feed a cat dog food. (But even dogs do better if the food is supplemented with it.) I believe cats are also more sensitive to sudden changes. So that's an issue even if it's cat food.

Most pets spend an entire lifetime eating less than optimal food. So I don't think a day or two until you can get to the store is a huge deal. Of course, keeping up on your shopping is a good idea. :-)