Depends on the size of the 'messenger bag'. BA is good in that they allow two carry-on items (with lesser airlines it's only 1), but both must conform to their individual specifications re: 'carry-on' and 'laptop'

http://www.britishairways.com/travel...n/public/en_ca

OH, and I have got them both in the overhead bins in the past too, but I only do that on quiet flights out of respect for my fellow passengers (i.e. the small one goes under the seat in front if it's busy...).