Bone to drive another bone's action

I know i’m probably losing it and should know this already, but…

I have a bone, which is in an action.

How do I get another bone to drive that action?

NB: 2.5


Told you i’m losing it…

Basically use an Action Constraint on the bone you want driven by it’s action, controlled by a named target controller… if that makes sense!

This was set as solved, but…

  • I set a keyshape to be driven by a bone (A).
  • Next I added a constraint to the bone (A) so that it was controlled by another bone (B).
  • When I moved bone (B), bone (A) moved correctly but didn’t drive the keyshape as expected.
  • The solution was to re-setup the keyshape to be driven by bone (B).

But is there a way for bone (A) to be constrained by bone (B) and affect the keyshape?