I'm currently using the Hunter's Axe and I preffer the two-handed version of the weapon. I had accepted the fact that using the two-hander meant that I was unable to use my pistol. That was until I fought the second boss(or at least the second one I found) Father Guacemole. He was also using the Hunter's Axe and when I got him to the phase where he uses the two-handed version of the axe I though at least I can get some respite from his blunderbuss. Boy was I ever wrong! Is it possible for the player to use a two-handed weapon and still quickly use their gun?
Firing your gun while using a two-handed trick weapon isn't typically possible, but there is at least one weapon that allows you to do this:
The Rifle Spear has a built-in gun function while in its two-handed mode. I believe that you shoot it just like a regular gun, with L2. The item description includes the line "it is the only trick weapon with an attached gun," so it's possible that it's the only weapon in the game that can do such a thing.
There are also a number of trick weapons that still only require one hand even in their transformed mode, such as the Saw Cleaver, Saw Spear, and Threaded Cane.
The only weapon that lets you do that is the rifle spear, it transforms into a two handed spear that also shoots blunderbuss bullets. Theres also the reiterpallasch which is like the rifle spear but instead transforms into a one handed riposte sword/pistol, this means you can actually carry two guns or carry something else in your left hand, like a torch.