In the meanwhile - after contacting a friend of mine - I refrained from using the booster module I had ordered. He told me about his experiences with such boosters: not only they produce high voltage peaks in the output when they are switched on but also one of them blew his Synscan AZEQ6 electronics when the booster 'exploded'. So not wanting to make the same mistake I'll be going to use my 12V/12A lead batteries again. Though Synscan is only showing about 11,2V but power light is not flashing even if both stepper motors are concurrently in use for a longer period. Alternatively I'll use a switching power supply offering enough voltage and amps.