Online poker venues normally forbid this behavior, and use heuristics to spot cheaters. However, I wonder whether it is really an advantage to sit on a table using multiple accounts. Wouldn't that just maximize earnings and losses, but not necessarily tip the game towards the former?
Sit on a (as in the same table) is cheating. You know the other hands and what cards are out. It is player collusion which alone is cheating.
If one hand is KK and another hand is QQ then you would not play the QQ knowing KK is out. Do you think that is fair to other players?
I might be on TT from middle position but not play it because I know both other T are out. Do you think that is fair to other players?
You could put a squeeze on a player knowing a folds comes to the hand you chose.
You could put in a raise and re-raise and push off like QQ where they may have stayed around for the same raise against a single opponent.
You could play like 55 and JT suited and basically double your odds of one hand hitting.
Now play separate tables that is just maximizing your win (or loss) rate. Play as many table as you can keep up with.
Yes it is a huge advantage. An acquaintance of mine used to have a partner and between them they would play eight games locking up 3 to 4 seats in each game. They did this for quite some time and were never busted by the "heuristics" used to spot cheaters. This person has since moved on to play poker for a living.
Without ever getting too sophisticated with collusion, no squeeze plays etc., you gain a good advantage simply by being aware of cards that are out of play. Having a king high flush and being aware that the ace to your suit is in the muck is a huge edge.
It is not difficult to cheat in this way, some would argue that if your going to succeed online this is the only way you will be able to beat the games.