A trigger of the form “if X happens then Y happens” seems, to me, to indicate that Y happens once—and in this case, that Y is the opportunity to spend 2 points from the arcane pool, once, for an increase by 1 in the critical multiplier. But, the wording is just ambiguous enough that I could see interpreting Y as something like “you gain the ability to spend 2 points...” and then argue that you can use that ability as many times as you want in this moment. So the wording is ambiguous on whether or not the ability can be used repeatedly on a given critical hit.
But normally, if intended to be used repeatedly, the wording would be “for every 2 points from his arcane pool spent, his weapon’s critical multiplier increases by 1.” That would eliminate any ambiguity, and that’s wording that Paizo uses often. The fact that they did not here—even though the wording they did use is ambiguous—is strong circumstantial evidence against interpreting things so that you can use that ability repeatedly.
If you successfully make that case, you can make a RAW argument that an “increase” is not a “bonus” and thus is not subject to the bonus-stacking rules. This, to me, is even more dubious, since quite frankly Pathfinder isn’t that careful about its wording and that is some fine splitting of hairs. But you could make the argument. And if your GM agrees with both arguments, you could spend \$2x\$ points in order to increase your multiplier by \$x\$.
I really, really doubt you are going to find any GM out there willing to let you do this. Even beyond how weak the arguments are (at least in my opinion), the more glaring issue is that this would be exceptionally imbalanced—letting a critical hit deal ×10 damage or something would cause huge problems in the game. So, on those grounds, I would not allow it as GM, and I doubt any other GM is going to, either.
