Mounted Mastery + Combined Might = ???

[Phillip0614]

Mounted Mastery (lvl 8 Order of the Sword Cavalier ability) states that, when making a Charge attack, you add your mount's full strength modifier to your own strength modifier when calculating damage done by the attack.

Combined Might (lvl 8 Mammoth Rider ability) states that, once per round when making a melee attack from astride your steed during a charge, you add half your steed's strength modifier to the damage roll in addition to your own.

So my question is...how would those two abilities combine? Would they stack? Would Mounted Mastery override Combined Might?

[Michael Sayre]

You can't add the same source twice. Since Mounted Mastery and Combined might are both giving you access to your mounts STR mod, they don't stack.

James Jacobs wrote:

Modifiers from ability scores aren't actually bonuses, strictly speaking. If they are, they're untyped bonuses—which means they stack with all other bonuses except themselves. Thus, if you have multiple things that say "Add your Dex modifier to this roll," you only get to add your Dex modifier once.