rx 480 can't detect one of two monitors

Good morning, Today I bought a Samsung C27F396 monitor to be fitted to the other Samsung S27E510C monitor. I connected both of them with HDMI cables to my R4X480 4Gb card, but only one works. The operating system and the AMD driver only detect one. Do not know what to do, some help? Sorry for wrong English but I'm Italian and I had to use the translator.