I guess all of us who use more than 2 devices to regularly play the game have seen this happening and were, shall I put it mildly, annoyed by the way it works. At this moment in time there is nothing preventing you from logging the game from another device, while you are engaged in a battle on the other. What happens then is as soon as you actually finish the battle (Hell, it even allows you to smash the gate - perfect irony), you are disconnected and get -31 trophies by default. This is very annoying and unfair, as apparently simply starting the connection process is enough to screw the game over.
Thus, I hereby propose to implement a very simple fix, which is simply an error event in case the game is already running, preventing the connection of the other device. Even a confirmation button is fine, I would be happy to have that jus to avoid the aforementioned situation. No need to discourage whatever sorts of abuse you thought of if there is a roadblock preventing such abuse imho.
P.S. This was already mentioned in a couple of places, but a formal single threaded suggestion was not created to the day. I am hereby rectifying this.