I believe there is a feat in Ultimate Combat that allows you to "two weapon fight" while wielding a ranged weapon in one hand and a melee weapon in the other. I belive it is called "sword and pistol."
The feat does not however, allow you to treat all the weapons you listed above as double weapons.
"Two Weapon Fighting" stipulates that each attack must come from a different hand so though you might be able to argue that you use non sling end of the sling staff to hit in melee, the 2 muskets you mentioned have their melee ends on the same end as the end that fires bullets.
For example a Halbred has a hammer head, Axe head and a spear head, but you are only allowed one attack because they are all on the same end. A double axe on the other hand only has 2 axe heads but is a double weapon because both heads are on opposite ends of the weapon.
The sling staff says you treat it as a Club of its size when you melee with it making it a one handed weapon... but if I were your GM I would allow you to use the choke up ability the polearm fighter has to use it as a double weapon. Other wise I would have you wield a sling staff in each hand to two weapon fight.
Edit:
Come to think of it though...
As long as the Musket axe is a two-handed ranged weapon...it would mean you pull the trigger with one hand while the other hand holds the end that has the Axe head. In my mind this would make it work like a double weapon while using "Sword and Pistol"... So I would allow it.
See my edit above, but yes, one has to treat both the Melee part and the range part of the weapon separatly for the purpose of encantments for balance reasons.
A two weapon fighter has to enchant both of the weapons he uses, a person using a double weapon has to enchant both ends of his double weapon, so I can easily follow the rule saying someone who buys a weapon that has a dedicated part for ranged attacks and a part for melee attacks having to enchant both of those parts even if he cant two weapon fight with them yet.