Can machine gunners use 2 machine guns if I unlock the second weapon slot?
yes, though it would be inefficient since both lmgs will draw from the same ammo pool if they use the same magazines (like mg34 and mg42 or PPDs and PPSH)
never used that, I always stick with the backpacks, however I have seen ppl go for double primary weapons, especially machine guns, because it lets you switch weapons rather fast instead of reloading.
It’s generally better to go with just the one gun and bring extra ammo. You can always pickup a second gun when one of your other machine gunners inevitably die since the limit is only in the menu.
so both guns draw from the same ammo pool?
Then ammo pack would be a better choice, I guess.
I think I read somewhere that they will start to fix encumbrance as well, so if you carry two MGs your soldier will be practically walking or potentially crouching (under the weight) everywhere 
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Yes.
Or can take the perk that allows that from start or. When the other machine gunner dies, you pick up his weapon.
Only advantage is that instead of taking the time to reload, you just swap to second one and carry on shooting
Yes but is usless bot use the same ammo pool, if you want more ammo buy a large ammopouch, if you need less reload time do the first mg upgrade and took the reload perk
I agree, I tested it, totally useless, thanks guys.