If you have 2 people left, and one asks to check it down, openly, I don't see any problem with that. The other person has no obligation to honor that. They could keep checking it down, until the last possible moment for action, and then that player pushes all-in if they want to. Neither is bound to honor 'check it down?'
Since they are the only two left active, and are open about it for all to see, what is the problem?
There were others in the hand, they were just not currently connected.
If they were colluding in private pretending to not be doing so I would agree, that's a problem and needs to be dealt with, but openly agreeing to check it down, I don't think counts as 'colluding'.
This would not be tolerated in any poker tournament I am aware of. Where have you seen this done in tournaments?
This kind of table talk I have seen lots of times at real tables. Most of the time when chopping blinds, where the last two remaining players ask and agree to an action.
Chopping blinds in a cash game has nothing to do with explicit collusion in a tournament, and is between the blinds. Some people routinely agree to do it if it folds around to the blinds, others don't. It's nobody's business who isn't in the hand. Apples and oranges.