Initiative.
Celerity has an Immediate action casting time, and you cannot take Immediate actions while flat-footed. So whoever ceases to be flat-footed first can cast it first. Hence, it comes down to initiative under most circumstances.What happens if two wizards have celerity and they both want to cast it at the beginning of the battle? How do you decide who goes first?
A game of three dragon ante.
If both of the wizards have a "never flatfooted" ability of some sort, then the wizard who declares the use of Celerity last goes first (as it interrupts other actions).
That's one way to describe two wizards coming together to duel. Magic, gathering....Oh crap it's Magic the gathering...