Shield of Bashing and Shield Master

[Karjak Rustscale]

so, I did a quick perusing of the messageboards and didn't quite come up with the answer to my question, so here it is:

when using shield master (treating the shield's enchancement bonus as it's weapon bonus)

SRD wrote:

You do not suffer any penalties on attack rolls made with a shield while you are wielding another weapon. Add your shield’s enhancement bonus to attack and damage rolls made with the shield as if it were a weapon enhancement bonus.

and you get a +1 Shield of Bashing (treated as two sizes larger for damage, and treated as a +1 weapon)

do the effects of bashing and shield master stack?
so is it a +1 weapon that does damage as if it were two sizes larger, or is it a +2 weapon when used to bash?

I'm thinking no, but I might as well ask the mighty Rules forum for opinions. (the two sizes larger to damage is good enough for a +1 enhancement cost)

thank you for your imput.

[Starglim]

Enhancement bonuses to the same quality don't stack unless a specific game ability says that they do (see the paladin and magus class abilities, for example). It's a +1 weapon.