It all depends on how risky you want to play the game. Do you log off from one while doing a ride for the other? Do you log on 5 minutes away from drop-off to start looking for your next ride? Do you actually accept rides while doing a ride for the other? From many anecdotes I've read, this one is apparently extremely common. I'm sure Uber has some heuristics to discourage this behavior, but ultimately, unless Uber doesn't want the driver's business entirely, then drivers who are driven will find a way to game the system as best they can. Like always, the sweet spot is somewhere in-between.