You are absolutely correct sir!
Thanks a lot for your help - now it works like a charm. I've used Neuron's RS485 for communication with xS11 and separate USB dongle with CH340C chip (a bargain really, $2.7 from CZ shop) for communication with PRO1 energy meter through the same Node-RED setup getting data from /dev/ttyUSB0.
It's a pity that I cannot edit my previous post ^^ - feel free to edit it with a big warning that you cannot combine multiple processes on one serial (which is absolutely logical, but for us: HW newbies not obvious).