Does Mythic Power Attack stack with Two-Handed Fighter Greater Power Attack?

[Bbear]

My guess is it does not, but want to confirm.

Normally, Power Attack is -1 BAB, +2 dmg every 4 levels
Using a 2 handed weapon, it is -1 BAB, +3 dmg every 4 levels
Mythic Power Atk is -1BAB, +3 dmg, but using a 2 handed weapon becomes -1BAB, +5 dmg

For the Two-Handed Fighter, at level 15, he gets Greater Power Attack

"At 15th level, when using Power Attack with a two-handed melee weapon, the bonus damage from Power Attack is doubled (+100%) instead of increased by half (+50%)."

Using a 2 handed weapon, it is -1 BAB, +2 dmg +100% = +4 dmg

Now does Mythic Power for a Two-Handed Fighter using a 2 handed weapon becomes:
-1 BAB, +5dmg + 100% = +10 dmg?
or only the highest count
-1 BAB, +5dmg?
or some unintended game mechanics where they added the two effect together:
-1 BAB, +5dmg + 2dmg = +7 dmg?

[ExcaliburV]

It should stack, but I think the third calculation is the accurate one. It's not adding a +100%, cause there was already a +50% (strength and a half for two-handed weapons). So it would effectively only be adding another +50%

It wouldn't go from -1/+5 to -1/+10, it would be more something like -1/+7. There's a chance it would even round up to -1/+8, but I'm not sure.

Granted this is just me talking, I haven't tested it so I don't really know.

[Alistairetheblu]

Just tested; it stacks but at +6. Figure it goes 2+50% for 2-handed, +50% for this ability to a total of +4 (2+100%) and then adds either a flat 2 or a 50% bonus for the mythic feat.

[dataseer21]

I don't know off the top of but my two handed fighter at around 16 got a +30 if that makes sense. It was a -5 to attack. So that would be a +6 as the comment above me says.