See: Thunderstriker, Shield Master
The question I wish to ask is whether or not the Armour Enhancement Bonus from a +2 Shield would double up from the Knockback Smash class ability and the Shield Master feat?
Shield Master grants a Weapon Enhancement Bonus, where as Knockback Smash seems to grant an untyped bonus to attack and damage rolls (that happens to equal the buckler's Shield Enhancement Bonus), so they would indeed stack.
EDIT: There may be an issue of the "source(s)" of the different bonuses. Since bonuses from the same source do not stack, if the source of all the bonuses is the shield itself, they would not stack; however if the feat and class feature are the sources of the bonuses, they should stack.
I would say they stack but do not increase each other.
Cheers! Thanks again for the prompt reply!