Melee firearms in action.

[Maxximilius]

Because your off-hand weapon has to be the one-handed axe for the purposes of TWFing. There is no rule for using a two-handed weapon as an off-hand one.
But actually, I know you can attack with either weapon first when TWFing... I don't remember if this choice remains in effect to decide which weapon is main or off-hand. Same thing, an alchemist with three or four hands may TWF with TH weapons, the off-hand TH weapon just gains the same 50% bonus damage condition... I guess you could TWF with the musket and the axe, with the musket as your off-hand attack as to not lose the advantages of a two-handed axe main weapon.

Note that you may also instead make your first attack with either weapon, and alternate until you made all attacks given from your BAB - there is no penalty induced as long as you don't get an additional attack during the round from this weapon swap.

[Maxximilius]

ossian666 wrote:

It is considered a double weapon for purposes of creating masterwork or magical versions of this weapon.

Only for these purposes, not for handling the weapon, otherwise the penalty would be -2/-2 with the feat.

[blackbloodtroll]

It is still two weapons. Just as the pistol dagger is, which, it as well, can be used to two weapon fight.

[Maxximilius]

Indeed, but two weapons =/= double weapon.

[blackbloodtroll]

That is true. You may not be able to two weapon fight as if wielding a double weapon, but you can two weapon fight as if you had two weapons, because you do.