Hamper Magic ACF vs Multiclass Caster?

[Road_Runner]

So we had this discussion in our group about about how the Hamper Magic ACF for Spellthieves functions against multiclass casters (a.k.a multiple different caster levels).

To hamper magic, you must make a successful sneak attack and give up 1d6 or more points of sneak attack damage. Against a target with spells or spell-like abilities, hamper magic reduces the target's caster level by 1 per 3 spellthief levels you posses. A spellcaster whose caster level is reduced to 0 cannot cast any spells.
Against a target with supernatural abilities, hamper magic forces the target to make a Will save (DC 10 + your Charisma modifier + half your spellthief level). Ona failed save, the target can't use supernatural abilities. Both effects last for 1 round per spellthief leve

I assumed that this functions against all caster levels of the target you used it against. For example if a level 9 spellthief sneak attacks a level 10 wizard/level 8 sorcerer, that creature would take -3 reduction in caster level for both wizard and sorcerer levels. My friend assumed that this functions like the steal spell ability in that you can only steal 1 spell from 1 class, so therefore you can only damage the caster level of 1 class. I don't see how this could possibly be how it functions because there are no rules regarding which class would take the reduction....

Obviously its all up to the DM but I was curious how the forum would interpret it.