Do you mean that you first "short" the stock at $10, then.....
If that is true, then you would want to enter an order cancels order (OCO). Buy X shares at $8.00 OCO Buy X shares at $12 Stop. If it trades at $8 first, it will cover your short and cancel the stop order. If the stock goes up first, then you will buy the stock back at the market when it touches $12. You won't be guaranteed $12. Your limit order at $8.00 will be cancelled when the stop is triggered.