Update: I think I may have found the problem for those of you that can't enable a second monitor using HD530.
Since the recent release of 10.11.5, when I updated I lost the ability to hot plug my second monitor at the desktop. The only difference on my system is, before I was using the Nvidia GTX970 and to use this with an iMac17,1 system definition I had to disable Apple's Graphics Device policy. This was disabled when testing HD530/dual monitor in 10.11.4 and was unaware it affected HD530.
Using HD530 with an iMac17,1 limits you to one boot display and no second monitor unless you remove Apple Graphics Device policy. HD530 is restricted too as is the Nvidia cards with this system definition. Now I've disabled Apples graphics display policy in 10.11.5 hot plugging my second monitor is working.
If anyone wants to test/confirm my theory go ahead and apply the fix in Problem 4/Method 2 here :
Solving NVIDIA Driver Install & Loading Problems Restart required.
Alternatively use shilohh's app at the bottom of Post#1 here:
Black Screen with MacPro 6,1 or iMac 15 or iMac 17 System Definition Restart required.