My stoker uses a top tube bag tucked under the stoker stem. A short captain puts the stoker stem too low for this to work. Anything too wide between my saddle and the stoker bars clashes with my stoker’s hands when they are on the tops. We have a tiny tool/tube bag tucked under my saddle for flat repairs. Other repair stuff (second tube, spare tyre and cables, etc.) goes into my handlebar bag or the saddlebag during brevets. My stoker can get food and other stuff out of the saddlebag side pockets without stopping pedalling.
There isn’t enough room around my hamstrings for a wide bag under the stoker bars unless it angles towards the stoker. Then the stoker’s knees hit the bag when we are climbing out of the saddle.
Stokers tend to load up my jersey pockets with all sorts of things, including food. If I try to get something out of my pockets, I tend to elbow the stoker, so I rely on the ‘in-flight trolley dolly’ (usually a bloke) for that sort of thing.