Can you interrupt the standard action a wizard acquired via celerity with a celerity of your own?

The answers in the Question: A question about Celerity spells confused me…

Some people claim that immediate actions are resolved in the order they are called wich I agree, except in the case of celerity because it says you can interrupt with it:

The Celerity spell states:

(…)When you cast this spell, you can immediately take a standard action, as if you had readied an action. You can even interrupt another creature’s turn when you cast this spell(…)

Immediate actions:

Immediate Actions Much like a swift action , an immediate action consumes a very small amount of time, but represents a larger expenditure of effort and energy than a free action . However, unlike a swift action, an immediate action can be performed at any time — even if it’s not your turn. Casting feather fall is an immediate action, since the spell can be cast at any time. Using an immediate action on your turn is the same as using a swift action, and counts as your swift action for that turn. You cannot use another immediate action or a swift action until after your next turn if you have used an immediate action when it is not currently your turn (effectively, using an immediate action before your turn is equivalent to using your swift action for the coming turn). You also cannot use an immediate action if you are flat-footed .

So with this info let’s say a fighter wants to attack you, you have to call your celerity before the attack roll right? If a mage wants to cast a spell, you have to call your celerity before the spell benefits/roll etc. is called (to interrupt them of course), why can’t you do the same while the wizard using celerity (let’s say not on his turn), you could just interrupt his standard action (given by celerity) with celerity?

I’d like a RAW reference if possible (FAQ or pathfinder is acceptable if there’s nothing in 3.X)