Is this a legal combination?
Nope. They both modify armor proficiency, so they do not.
They also both modify Divine Bond, so they don't for that reason too.
Crappy, the two archetypes seemed to go hand in hand... :/. Guess it might work to go Cavalier musketeer/luring cavalierX and Paladin multiclass.
The problem is that Holy Gun and Divine Hunter are both terrible Archetypes, even if you were allowed to play them. Divine Hunter replaces too many good abilities with completely useless ones, and Holy Gun has the oddest written and worst possible replacement for Smite Evil while also hurting your proficiencies. My advice is to go Gunslinger 1(Mysterious Strange Archetype)/Paladin(Oath of Vengeance). Mysterious Strange uses CHA for grit and by spending 1 Grit you get CHA toattacks to go along with your smite, and Oath of Vengeance gives you a TON of smites. This effecively makes you Joshua Graham, so feel free to play the scariest Holy Man ever. If you really need the bonus feat at level 1 to get Precise Shot, Oath of Vengeance still stacks with Divine Hunter.
An issue is keeping your gun maintained as a Mysterious Stranger is that one of their abilities replaces Quick Clear, so keeping some scrolls of Make Whole are needed after a certain level. They should cost 125 each. Another option once you can afford it is to get Reliable enchanted onto your gun to reduce it's misfire chance.