Allied caster stacks? Bug or working as intended?

[Iluvcheezcake]

So I am running a full caster party with all chars having spell pen / greater spell pen / mythical spell pen.
Atm lvl 11, and sometimes having trouble with overcoming spell resistance.
I respecced with the ingame respecc dude (0 mods) and took the Allied caster feat on all casters and all of a sudden i get over 30 bonus + the rolls when calculating mob spell resist chance (without the feat it was something like +17 or such).
Does that mean Allied caster stacks the same way Shake it off does or is it a bug?