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?
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 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.
If those two were doing so consistently, then maybe it would be worth investigating.
My 2 cents...
-- Smoov
This all depends on the people, really. In RL games I've seen times where the last 3-4 people in a tournament would just agree to split the pot according to estimated ratios. Some will split evenly. Sometimes people get infuriated at the idea of it.
Something I've learned from playing is that even though you may experience something being accepted in some rooms, others may have rules against it.
I agree, by the way, that agreeing between two people to check it down would NOT be colluding.
For the record, I got in trouble in one room because I was heads-up against someone and went all-in. He was taking forever to make his decision so I told him what I had and flipped over my cards to make it easier. I ended up being force-folded for it, although it was just me and him anyways.