can I use two stinger garage door opener transmitters for the same door?

yes, just code them the same

YES ..... one opener [ remote control per car ] and both of those stingers must be on the same frequency as the controller that is at the door-opener receiver located on the door up-motor == all three of those devices must have the same CODE to work properly and the way you want .... be sure that the garage ceiling box and both car controllers are on the same radio control wave === open the two car buttons and check for frequency and code .....