Whenever duoing the person of lower rank will always have the advantage between the partner due to the fact that if he loses, he will lose a normal amount of lp and his mmr will be unchanged but, the higher rank person will be playing against lower rank players so a loss will result in a larger amount of lp lost and mmr drop and either way both partners gain the same amount of lp so when duoing only the lower ranked player gets the advantage in most ways the higher ranked player is doing the lower rank a favor by playing with them. Also when you are duoing the game on purposely "better" kids or kids with decently high winrates and they match you with kids with negative or just plain out worse then the other team on your team. They do this on purpose their logic is if you duo with someone you should be responsible in carrying the game cause, you have advantages like 3rd party communication like curse or skype and you can coordinate better.
TL:DR The lower ranked player benefits alot and the higher ranked player benefits minimally. Dont duo unless you are confident that you and your partner can carry games together.