Hi Michael, I understand you have a 6 ft mDP to DP extension cable? When using a DP cable it’s very important to have one which supports HBR2 (High BitRate 2) to benefit from the full DP 1.2 bandwidth. Many DP cables are RBR (Reduced BitRate) which means they do not support the full bandwidth which is required to power multiple monitors. This will result in a bad signal/ flickering etc. 

Some more information on DP cables can be found here: http://www.displayport.org/cables/how-to-choose-a-displayport-cable-and-not-get-a-bad-one/ 

We offer the CAC-1064 DP extension cable, combined with our CAC-1110 mDP to DP adapter your setup should work: 

http://club-3d.com/index.php/products/reader.en/product/mini-displayport-to-displayport-cable.html

http://club-3d.com/index.php/products/reader.en/product/displayport-cable-ver-12.html