Robin2,
Thanks for the reply. You are right, it's not HC-12 for 315MHz, rather "XD-FST XD-RF-5V kit" (e.g. http://www.banggood.com/315MHz-XDFST-XDRF5V-Wireless-Transmitter-Receiver-Module-p-925524.html) which looks and operates literally the same.
I mean, if it had to be a separate devise for 315 - I wouldn't even have to change a line in the code, but to replace hc-12 with XD-RF. But the idea is to connect both (and listen to both at once) with one Arduino. Is it even possible with UNO, in terms of hardware?
Thanks!