Two sticky pistons in a horizontal fashion. placed so that one holds the other. Two buttons, one triggering the one holding and the other one triggering the one being held. Both buttons do as they should however if you extend the piston being held and then push the button of the piston holding at this moment the piston being held will remain extended, the piston holding will not extend.
It seems only to work or indeed not work if the button that is used to "lock" the piston being held into the extended position is placed above the piston holding the other piston.
In the picture: Extend piston using button 1. While piston 1 is out press button 2 . Piston 2 does not go out but piston 1 stays out.