1.If you are using two VGA to HDMi converters, you must try one DVI to VGA or DP to VGA adapter + one of yours VGA to HDMi converters
Try driver reinstallation:
2.Leave only one monitor for this operation
3.Delete C:\AMD folder
5.Then reboot and plug your second monitor and check that win10 has installed correct driver for each monitor
Today i bought Sapphire RX480 Nitro+ 8GB.When i am playing on my pc monitor(lg 1080p via HDMI2.0) everything is great(The card is really powerfull at 1080p.There is no game that can stress this card@1080).But when i am trying to connect a second monitor(samsung ku6000 4k 43" via HDMI2.0) everything just falling apart.First i had the same problem with you.I tried a diferrent HDMI port on my TV and the flickering stopped.But there are more issues.Until today i was using an nvidia card(gtx660ti and i was able to connect my 4k tv with no issues(3840x2160,hdr enabled(10bit),60hz,).With rx480 the reccomended resolution is 4096x20160,although my tv isn't supposed to support this resolution,hdr is not available,also I noticed some weird white pixels flickering at some points(due to refresh rate i suppose).At radeon additional settings i noticed that the maximum reported refresh rate was 75hz although i have set my TV at 60hz.When i am trying 4k gaming while both monitors are connected(via HDMI) some games stuck at loading(alt+enter sometimes helps).I found some workaround for some games(i put the monitors to extended mode,i start the game at pc screen and i change to 4K TV through the game's video options,but this isn't feasible with every game e.g Fifa).At some forums they suggest to use displayport instead of HDMI.I am gonna try it soon and i will post the results.