Some airlines are fussier that others - EasyJet says that you can take as heavy a carry on as you like provided it fits the dimensions and you're stong enough to stow it yourself - sensible for them as it means fewer passengers have to check baggage which is expensive for them to handle. Ryanair is even talking about charging everyone who checks in baggage a handling fee. However, I digress. Being longhaul everyone checks bags anyway with Virgin.
I've been stopped with a too-heavy carry on bag. If the flight is full, they tend to check. If you can balance them on one finger when they ask what carry ons you have, they tend not to check. There were two of us travelling and the suggestion was to transfer some of the stuff to the lighter carry-on of the second person, which we did. Does you bag exceed the 6kgs on its own? If not, can you "cram" Jamie's bag and then revert? Just a thought.
By the way - be aware that the checked baggage allowance is said to be two pieces of unlimited weight each, but in practise there's a 32 Kg limit per piece. I've been cuaght by that one too and had to move things around at checking into a box kindly supplied by Virgin. Interesting, the box was second hand and labelled "British Airways".
Hope these ramblings help you see your way to an answer!