depending on the new rules that this would entail, it could also roughly double your opponent's ability to checkmate you, also a single fork would do the trick (double check), so it wouldn't be a good idea unless both kings should be checkmated at once for a win, in which case I don't know... can you give up a king? if not then your opponent could check you at every move (or at least, doubly as often) and the only thing you can do is draw or resign... sounds kinda boring. So nope I'd get a queen instead 
If it was within the rules, would you ever consider promoting your pawn to a king?
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Think about it, now you ahve two kings, your opponent would have to # you twice to win.