The AND controller is your solution here.
The AND controller makes it so that the sensors attaching it ALL have to be sending their triggers at once. So if you have two keyboard sensors, say the keys “A” and “B”, attached to the AND controller and then that AND controller hooked up to a Motion actuator, then the motion actuator will only trigger if both the “A” and “B” buttons are pressed at the same time.
Here you’d like to create either a “Touch” or “Collision” actuator. For the touch actuator, you’ll have to add a material to your ground object, call it something like “ground” or anything, and then put that into the sensor. Then attach that to the controller that your keyboard sensor is going through and… vuala!
One more note… you’ll probably want to use the “linV” kind of motion, as Force won’t work that well with this kind of setup.