so if i do rogue into a musketeer will it show the dagger along with the riffle?
I’m pretty sure muskets are 2-handed weapons. 
Yes, you can. But not both at the same time. You gotta swap weapons for such. (You can pretend youre using both with Bayonet Thrust, though. lol)
Musket or rifle as you call it for Musketeer class is 2-handed. So you can’t equip a dagger on the off-hand together.
But when you get Weapon swap you get like 2 seperate equipments slots for weapons. You can equip dagger on the other weapon slot and swap for it.