^,^ That's about what I figured, brimic
Master keys for an "infinite" number of tumbler setups would seem to be pretty awkward.
About two years ago the managers here installed deadbolts on all the downstairs apartments. I don't know if there was an incident which precipitated this or not. I already had a deadbolt on my second floor door, installed even before I rented the place.
In any case, I figure the probability of one doorknob key fitting another one would be one in five, or 0.20. I guess it would be similar for the deadbolts.
Anyhow, figuring that way, per brimic, I guess the chances of a person's two keys (knob and deadbolt) being able to open any other given door would be 0.20 X 0.20 = 0.04, which is a lot better odds, or 1 in 25.
If my reckoning is correct for mechanical keys.
Terry