Create a rule that 2 cards in a list

[Oti roa design]

Hello Trello 

I would like to create a rule that doesn't allow 2 cards in a list or when someone move the 2nd card in a list it will become red or yellow showing the user that he can't have 2 cards in a list at the same time 

[Dreamsuite Mike]

Simplest way to visually notify is to use the List Limits powerup 

You can go further and use automation to move it back to where it came from.

You could even add another operation to post a message to the member who moved it saying, "Uh oh, who's been naughty...." or something along those lines