As there are a few mac gurus on this thread - an OT question... colleague has 2014 rMBP and used HDMI out and a DVI adapter via thunderbolt port to have two external displays.

Sierra update killed that - only one display will work at a time.

prams zapped, settings fiddled with, cables tested in millions of combos and even playing with AppleGraphicsControl.kext

Any other ideas? Seems nuts an update can kill such a thing!