Definitive answer here, from Nicotinelle, the producers of the nicotine patches I’m using currently.
They are intended both for quitting completely and for cutting down the number of cigarettes you smoke. Provided you use the stage 2 patches (around 14/15mg nicotine) not the stage 1 (25mg) it is safe to smoke whilst using a patch, but you should smoke fewer than you do without the patch.
I’ve cut down from 10–12 a day to 3, with the patches, in less than a week.