First of all.
Duo Q Boosting is not Account Sharing.
It works this way.
You pay a high Elo Booster,who will play on a Smurf (most are botted Accounts), to play ranked Duo with you. On this way you play on your own Account and still get boosted.
I feel that this is unfair (esp. The pay for it part) but i dont find sth in the TOS against it.
I know this is in reality very unlikely that you get a ban for that. (Cause its hard to detected). Its more about the theoretical part.