Who decides first, straddler or poster?

[Glurfle]

Situation: some guy's been away from the table and posts both blinds in the CO. UTG sees this and decides to straddle. CO now decides he doesn't want to post if UTG is going to straddle. UTG then says he doesn't want to straddle if CO isn't going to post...

Alternatively, you could construct the same sort of situation with multiple posters, or someone choosing to take the big blind contingent on a particular person posting, etc. Is there a standard way to deal with this situation?