Assuming you mean the Musket Master archetype? No, you can't multiclass between Gunslinger and Gunslinger (Musket Master). A Musket Master is a Gunslinger, just with a slightly different set of class features.

If you want to use a musket, you'll really want the Archetype. Rapid Reload at first level is a godsent for gunslinger, Fast Musket at third is super-important if you want to actually attack regularly, and Musket Training is actually more versatile than Gun training, since it works for all two-handed firearms. You can get pistols back with either a feat or-- and this would be my choice-- a level in Swashbuckler with the Picaroon archetype, which also nails you a bit of extra Grit.