5K Display (only one) and MacBook Pro mid-2015 DG Radeon 370x
Per the technical specifications, your MacBook will support a maximum resolution of 3840x2160 which is technically UHD 4K (16:9)

Can you connect a 5K display?

Yes, but it will show at the 4K resolution if you use one Thunderbolt cable.  If you use two, you can drive a 5K and the display must support MST (Multi Stream Transport).  In this scenario, only one external display is supported.

See Using 4K displays, 5K displays, and Ultra HD TVs with your Mac for more details.

Bottom line: you can only display up to the maximum that your source can push.  In other words, you can drive two monitors at 4K or one at 5K