Equipment: Early 2015 MacBook Pro - Retina 13" with the Intel Iris 6100 graphics. Two Dell U2414H monitors. One MiniDP to DP cable connecting MBP to monitor #1. DP to DP cable connecting monitor #1 to monitor #2. OS X 10.11.6.
History: According to the specs for the Early 2015 13" MBPr the laptop is capable of supporting "Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 3840 by 2160 pixels on up to two external displays, both at millions of colors."
According to Apple Support article HT206587 the video card does "support multi-stream transport (MST) displays at 60 Hz".
According to http://www.displayport.org/cables/driving-multiple-displays-from-a-single-displayport-output/ support of MST 1.2 means the laptop should provide "the ability to stream independent video displays from a single desktop or notebook video output".
Lastly, according to https://software.intel.com/en-us/articles/quick-reference-guide-to-intel-processor-graphics the video card is in fact capable of supporting DisplayPort 1.2.
Problem: When I connect both monitors to the MBPr and "Enable 1.2" on monitor #1, the screen from #1 is mirrored on monitor #2. After doing some research, it appears that I need to change a setting to switch from mirrored mode to extended mode in order to have two different screens. I did Enable 1.2 on monitor #2 as a test but there was no change.
Support: I called Apple and was told to call Dell. I called Dell and the tech informed me that it was definitely a problem with the source and explained how on a Dell PC you could switch between mirror and extended modes within the graphics settings. I again called Apple and spoke with a senior advisor. I was first told that it could only be done using Thunderbolt cables. I directed his attention to the support article and said the listed Dell monitors did not have Thunderbolt. I was then told my video card didn't support that feature but explained that he was referencing the AMD Radeon R9 M370X card which is not my card. He said he would escalate the issue.
Question: How do I enable extended mode?