I thought about this after reading the Thomson air thread. Some of the airlines do make you combine everything together for one carry-on whereas others allow you to bring an extra rucksack etc. I was pleasantly surprised on a recent Paris to Fez flight where Easy Jet actually gave me a free 2nd check in bag!
But shockingly on the same trip spring trip, Air France on the Paris to Amsterdam route made me cram everything in one bag (managed and put some extra things in my jacket pockets) even though I don't remember reading anything about one carry-on bag limits with Air France before.
I do remember flying with Ryan Air out of Gatwick where they weighed our carry ons just before we boarded and both my wife and I had overweight luggage. We probably should have tried to carry more in our pockets (can't remember if I had the same rain jacket with the huge pockets in my carry on or not), but instead just ended up paying (though I still think I should have tried to at least get one bag underweight, but I seem to remember we were both over more than a bit). This is the reason I think its sometimes better to choose a full service carrier (hence why I was surprised by Air France's request) or take the train (like we're doing this year from Brussels to London, London to Cornwall - return and London to Paris).