It is completely possible to fire your musket (repeatedly, if needed) and then use your sword and/or pistol in direct combat, or vice versa as mentioned. In some cases, it can be a devastating one shot kill. It should make sense you cannot draw your sword when using a two handed weapon, as there is no "slinging" firearms in this game.
Here are the steps in more detail on HOW TO DO IT.
It requires you to unequip your musket (any type) via the inventory screen in mid combat.
You have to put away your musket using the "E" key (non combat mode), hit "F2", go to your inventory, unequip the musket entirely and then return to the fight, and draw your sword with the "E" key again. Reverse the process to reequip your musket. Not exactly optimal game design in the middle of a close battle, especially with single opponents, but you can if you are quick with four keystrokes and one mouse button click. There are however, game situations and quests where this is useful, and a few it is a necessity.
However, one additional note is you will only be allowed to use lead bullets (or a grapeshot pistol combo if you are using an Arquebus) for the weapons in question, as there is no way to "talk out loud" while in combat to switch to paper cartridges, or any other types of ammunition, and reloading time is still required regardless.
Final note, some muskets are better than other dependent on the situation, many unique weapons can be acquired via quests. Grapeshot is better for multiple opponents using the standard Arquebus, the Matchlock, "Screw Nipple" (Rifle Targeted Musket) or Side-by-Side Musket (two shots) are better for single opponents. The naval carbine has better use a melee weapon (while it is reloading) since it has a bayonet attached for higher damaged physical attacks. The hand mortar is great with multiple opponents and grenades, but requires special crafted ammunition prepared beforehand, and friendly fire can be a problem. There are few other choices, and combinations. If you are looking to quickly level up your firearm skills, use muskets. The best combo NOT using a musket is a sword and Blunderbuss pistol with nail shrapnel, but is not acquired until later in game.
It is pretty entertaining to clear out an entire enemy fort using just the hero, a musket, a LOT of ammunition, and very high firearms skill. But it can be done, if you want a challenge.
