That is police procedure. And another reason why civilians shouldn't use it as they don't have backup.
There's a few things wrong with using a mix of rounds:
1. People forget under stress and can forget what round is loaded. It's like people who carry pistols with an empty chamber and think "there's no way I'll forget to chamber a round". According to ASP, most people forget to rack the slide and some people died because of it.
2. If you have a less lethal round loaded, and you suddenly need a lethal round.
3. You shoot the less-lethal and kill the person you may get into trouble.
4. If you decide to use lethal force, the prosecutor or suing attorney can say you meant to use non-lethal when you used lethal.
That's why it's called less-lethal and not non-lethal. Even tasers have killed.
If you need a lethal round in a pump shotgun, it's only a racking or two away. You aren't required to discharge the less-lethals before getting to the man stoppers.
All good asvie about making things simpler for you so you don't make a mistake in a pinch. But that's tactical planning, not legal mandates.
if i shoot 2 LTL/LT rounds and then shoot a slug killing the attacker, it can be argued my intention was to merely stop the person, not to kill them. Only when it became necessary did I fire a lethal round.
As for not having one in the chamber, well that's not debatable in my mind. A gun without a round chambered is a conversation piece.
I had a KSG 14 with 2 tubes. I loaded slugs in one and buckshot in the other. In this hypothetical, it's a matter of flipping a switch to choose which one you want to send to your target. The nice thing is you have that option, whereas the mixture in a single tube becomes a "hope it plays out this way" method.