So, you've had both cats for practically their entire lives...
Do you know which one is dumping outside the box?
Is it the older one or the younger one?
If it's the older one... there could be a medical problem...
If it's the younger one... or older one... it may be a "finally time to assert dominance" thing.
Cats use scent to mark territory and to assert dominance in that territory.
Burying their poop is essentially a submissive/social "democracy" gesture.
Something may have happened recently (think about it... you may be able to put a finger on it) that caused one cat or the other to either feel threatened, or to feel that the other cat was now less able to assert their previously dominant position... this could have led to one of them dumping outside the litter box as a way to claim or reassert previously 'owned' territory...
Note that this may be entirely separate from and seemingly unrelated to how they interact either in front of you or otherwise... we're talking about instinct here... nothing like human emotions or rationales are involved.
It's entirely possible that one or the other of your cats has sensed something to trigger such an instinct, yet have it only be manifested in dumping outside the box, and not involve any other interaction that you can observe (like them not getting along anymore).
It could have been a change in your lifestyle, or theirs, or both.
Is the outside-the-box dumping always at the same location?
If "yes", try putting a bowl of dry food at that spot... see what happens.
Cats are like the Mafia in one respect... they won't s*h*i*t where they eat!
Just on the outside chance...
Are you completely sure that there isn't some intruder cat sneaking into your basement and leaving extra-litterboxial deposits? I'm serious.
Check the windows, etc...
Good luck! Keep us posted!