Why retina mac book pro won't output signal to second monitor via mini display, but detect it?
Your model should support 2 external monitors at max resolution of 2560x1600.

Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on up to two external displays, both at millions of colors.

Check your resolution settings, if the cables are working.

https://support.apple.com/kb/SP704?locale=en_US