Your program may work well unless you press two switches at the same time. Then the motors will receive contradictory commands from different threads. The same will happen if a switch is not well connected.
But in general it is not good programming practice to use threads in this way. Threads are meant for processes that should run really independent of each other. It are in fact different programs running at the same time. Here the threads are dependent of each other: one thread can say to the motor "turn left" and another thread can say at the same time to the same motor "turn right". No wonder the motor gets nervous.
I think something like this is better (and simplier):
If you press here two switches at the same time, the program will only see the first one. In the same program you can add the other part (rauf/runter), no extra thread needed.
Regards, Han.