Different speeds for multiple motors

Hi,
How can the speed of 1 motor be changed while 2 motors are running?
Switching off one motor is not very nice.
pwm-duty changes all motors instead of one only (pity!).
The code I use is based on the example of 
~\punyforth-master\arch\esp8266\forth\examples\example-geekcreit-rctank.forth
Thanks in advance, Jos