Thanks for the offer, but don't worry about it for now. I'll test later this afternoon or tomorrow and report back. On site in the dome shown in the original post was not the right place to debug things while the client was watching, I'm sure I'll be able to isolate the issue in the more relaxed home environment. :-)
I will check again later today with my two external LCD displays.
But I can confirm, I had no trouble with the internal and one external display from my laptop, the issue only occurred with two external displays.