This talent gives just +5 on the test, which doesnt feel enough (and feels odd to do a coordinated interrogation alone).

Yes - it also gives +5 for each other acolyte with this talent, but this would mean that every supporting acolyte also needs this talent...very inefficient.

Why not give +5 for every other acolyte in general and even +10 if he also has the talent ?

As it is "coordinated", there shouldnt even be a bonus at all for interrogating alone.

I feel the benefit is strong -- if a whole party of four acolytes purchases the talent, the lead interrogator gets a +20 bonus to their Interrogation check.

However, the talent is too expensive for the benefit it provides. It's a tier 2 talent with the Intelligence and Social aptitudes -- a 900xp purchase for someone without either aptitude. I think it should either be reduced to a tier 1 talent or have one of the aptitudes replaced with General to lower the cost. It might even be appropriate to do both -- this way, a Warrior with no social aptitudes whatsoever could purchase the talent for a reasonable but not insignificant 300xp and contribute something in non-combat situations.

You could always have each member of the coordinated interrogation roll a skill test on something appropriate to determine how much they contribute.

Allow participation by those who don't have the talent under the coordinating acolyte.