My family also comes from Manchester and I was also told about the two brothers and the French revolution. However, I have traced my tree back to well before this and so I cannot be decended from one of the two brothers even if they existed, which I doubt. Another Quincey researcher, who is a distant cousin, has also heard the two brothers story and is equally sceptical. We are both descended from a William Quincey bap 1696 in Lincolnshire.
Thomas de Quincey adopted the de as was the fashion at the time. I was given to understand that the line died out with him but you may wish to check this with Dr Peter Quincey of The Guild of One Name Studies. I am presently in Cairo and can let you have his contact details upon my return to UK in March or you should be able to track him down through the Guild's website. The last time that I spoke to Peter he didn't have an e-mail address.