Let us be honest...
Fighter Devout is a very strong subclass because Penetration is HUGE so far in POE2. Too important really. This is balanced by only being able to be proficient in a single weapon. But when multiclassing, you can learn more weapons and still retain the huge bonus you get as if you have only selected a single weapon.
While I have been using this build myself, I must be honest in saying that it is extremely OP and should be removed. If you choose the 'Devout Subclass,' you should not receive more weapon proficiencies from your second class.