Hello CircutScoper, Good morning. Both circuits are working fine. The 2-relay circuit which I have built and to be switched over GND is functioning without the 2 diodes (diodes on right-hand side of my diagram) I became aware that I had switched mistakenly the brown cable to the pin 1 of the 2nd relays and the black on pin 2 of the same relay. I corrected this and all is functioning as it should be.
As you mentioned before, building the circuit with 2 relays is cost-expensive, I thought it should do the job using an IC such as the L293 or the CD754410. See https://nathandumont.com/blog/h-bridge-tutorial
So, I will experiment with this IC. I read an article in which was indicated that connecting a potmeter to EN1 of the IC, you could reduce speed of the motor which is running to fast in comparison with the 2nd motor.
I keep you informed about the progression