So the relevant specific text of the spell Disintegrate from Archive of Nethys reads as follows
You fire a green ray at your target. Make a spell attack. You deal 12d10 damage, and the target must attempt a basic Fortitude save. On a critical hit, treat the save result as one degree worse. A creature reduced to 0 HP is reduced to fine powder; its gear remains.
Now since this spell involves both an attack roll AND a Fortitude save, as far as I can tell, RAW this allows you two opportunities to multiply damage from the spell.
Whenever you make an attack roll, and you crit, you double the damage done by the attack. Now, Disintegrate specifies another effect that also happens on a crit, but nothing in the rules, unless I have missed something, states that this effect replaces the normal crit effect.
Then the recipient makes a basic Fortitude save. A crit fail on this also doubles the damage from the source. And since on a crit attack roll, the degree of success is dropped by one, that is quite a bit more likely to happen.
Thus, on a crit hit, followed by a crit fail on the Fort save (which you still get on a regular fail), Disintegrate SHOULD do 12d10 x 4 damage.
Is this the correct interpretation or in this instance is there a reason that only the Fort save considered when calculating damage multipliers?