Dental eugenol, it's a fairly powerful local analgesic made from clove oil. Some people have adverse reactions to it and so it isn't used as frequently as it used to. So I would recommend applying it in a small test area before applying it more liberally.
You can buy it online fairly cheaply.
For exterior doors....? More than likely those are meant for interior use, and someone is being cheap and installing them for exterior use.
Either that or they just appear to be affixed together, but will still operate the tumbler even if one is being held in place.
The reason they don't really use a single spindle anymore is because it makes them extremely easy to brute force. You can literally just knock the handle off and access and operate the latch assembly.