If I have 2 {frontier siege} out, both choosing khans, would that give me 4 green mana per main phase or would it be redundant?
4 green, yes. Each of those is specific to its own card so there's no reason why you shouldn't get 4 green. Not quite sure what ruling to even look for on this so someone else is welcome to do that for me :P
Well seeing as each card is it's own individual object, they'd trigger seperately and be like siege 1- mana
Siege 2- mana
They aren't the same object so they each trigger seperately. It's just like if I have 2 {Guttersnipe} on the field and cast {Doom blade} they'd take 2 damage from one 2 from another and if opponent casts {Pay no Heed} (or some other protection from specific source) they can't just say "Guttersnipe" and stop all the damage, they'd have to pick one to stop. And the other would hit