Q: Dual Display With Retina 5K

You need a Mini DisplayPort to Mini DisplayPort cable.

see > Use your iMac as a display with Target Display Mode - Apple Support

Not sure who told you that...?

Again you need a Mini DisplayPort to Mini DisplayPort cable, because that is what your 2010 27" iMac requires for TDM.

While the 2014 and 2015 Retina iMac's can not be used as a display for the Target Display Mode.... They can be used as a host system for both external monitors using Mini DisplayPort Adapters and TDM using a Mini DisplayPort to Mini DisplayPort cable.

Check the spec's > http://www.apple.com/imac-with-retina/specs/

Just to be on the safe side (before I break out my credit card), I can use my Dell U2711 27" as a secondary display provided I have a mini displayport cable?

i've used a dell 24" dvi and now an asus 27" dvi monitor as second display for my 2011 21" imac

both in OS X and in windows10 works like a charm

Yes I know "older" iMacs works perfectly, but since this is a Retina Mac, and since you cannot use it as an external display as you can with the previous iMac models, I thought I'd better be sure.