Hello thanks for your updates and efforts with translating into english.
I connected multiple unit steering cable between both EU07 locomotives. Otherwise I would have missed half of tractive force :D
My hypothesis was also that sudden loss of pressure in main reservoir (in my case as a result of unbraking trainset using high pressure pulse into brake pipe), which drained faster main reservoir on front locomotive than on rear locomotive and it triggered front locomotive's pressure sensor which turned on compressor, but rear locomotive's main reservoir never got below 7,5 bar and for that reason compressor on second locomotive remained inactive. But when I turned off compressors using C key, let the pressure drop below 7,5 bar for instance down to 5,5 bar and turned on again compressors by using C key, both compressors started to operate until 8,5 bar was reached. Sometimes when air pressure drop is very slow, for instance brake is set to running mode - 5 bar for long time and trainset is short, then both locomotives reservoir reach 7,5 bar threshold at same time and it works as designed in real life (probably?)
I assumed that multiple steering ("Sterowanie ukrotnione" in PL or in CZ "Mnohočlenné řízení") overrides pressure sensor on B locomotive and only sensor from A locomotive decides, if compressor should run or not. Simply said, that what does locomotive A, also does locomotive B with few exceptions. However when I submitted this question to Czech FB group "Strojvedoucí" I was given this answer "U EU07 ve dvojici kadá sama podle tlakového spínače." (Each locomotive switches the compressor independently according to its pressure sensor) So in case of EU07 this seems to be working as designed, but in case of ET42 this seems to me as legit bug