Both of these abilities deal Bludgeoning damage equal to strength "when you successfully grapple a creature".
Would they stack for 2xStr damage on a grab?
They both deal straight damage, so yes, they add on a Grapple (not on any grab---see below). To not stack they'd have to be typed bonuses to damage being dealt already, which of course Grapple doesn't do.
Would they trigger on Combat grab where you automatically "grab" a creature on a successful attack?
No, because a grab is not a Grapple. Grapple is a specific action by which you try to grab/restrain a foe. Combat Grab is a different way to grab them. For instance, Grapple has a size restriction and Combat Grab doesn't.