You might not need to tell them at all, anonymous, especially if their problem is less about the fact that they don’t know they can’t procreate and more about the fact that they want young and are willing to do whatever it takes to fulfill that need. That having been said, if letting them take care of your professor’s egg and resulting hatchling over a longer period of time is out of the question, then oftentimes, breeders end up with abandoned eggs, or a trainer decides they can’t take care of a hatchling. Either would be perfectly acceptable avenues for securing the egg or hatchling your pokémon want. Ideally, you may want to adopt an egg in order to give your espeon and umbreon the full experience of pokémon parenthood (short of laying the egg, of course), but if that’s not an option, allowing them to raise a hatchling together after it’s born works just as well.
Alternatively, you could use a surrogate ditto lay an egg or few for them so that the hatchling will at least be biologically one of the parents. Some trainers opt for that option, as it’s easier for the parents to bond with the egg, which is really the important thing. Your espeon and umbreon may not see the egg as theirs because it’s technically yours, as far as they’re concerned. If you, quite literally, give them an egg of their own, that should be sufficient to fulfill their wishes to be parents.
If, however, you’re referring to the possibility that they are mating with extreme vigor because they don’t understand that neither of them can lay an egg, then I would suggest explaining it to espeon first. Espeon, due to their psychic elemental, are more likely to understand human conversation than a dark-type umbreon, and in any case, it would be much easier to deal with the telepathic backlash from your espeon’s shock sooner, rather than later.
Best of luck, anonymous!
The Pokédex According to Bill 