Even if a similar thread was created not so long ago, maybe the little detail that can make a difference may not have been discussed. Well, I am wondering, if it is allowed (by law of course or isn't or wouldn't be unauthorized) to be on the two sides of a stock, if someone would buy or sell a stock in the US and also use an ADR to be theoretically on the other side. Would it also be not penalized to be on the antagonist sides, but by trading in two different countries at the same time? It is said that you can't place yourself on the same stock almost simultaneously, but an ADR has a a good ol' specialist to make you grey hairs grow before you get your order filled. Sometimes, the Devil is indeed in the little detail or lack of...
There's no rule saying you can't be on both the bid and the ask at the same time in the same stock. Unless you trade AMEX listed stocks, where they don't even veil their attempt to control their racket. Same goes for options.
In US you can't be both on the bid and on the ask side in options. The the second order will not be accepted. This is a market maker's privilege.
If you want to be on both sides, just use two accounts. I did, however, read once about someone sanctioned by the SEC for doing that with several accounts in order to pump up a stock.
It was a conspiracy among several people. They essentially traded among themselves, via the market, in order to create the impression that there was demand in the stock.