Because my sessions are only of 5/7 hour duration I only take one flask UNLESS it is really brass monkeys in which case I take two. As a young man I always took two with one holding tinned tomato soup and the other, tea, but this for longer sessions...
These days it is tea and coffee..The tea is always Assam teabags with milk and bags carried separately so I virtually have 'fresh' tea every time. Similarly, I carry foil sachets of coffee, the type found in hotels, which give one decent cup.
I do it this way so I carry the maximum amount of water in the flask and it stays HOT which is NOT the case if you add milk to the flask. In addition, I don't have to put up with the flask starting to smell from its contents and it only takes a minute to wash ready for the next outing.
May seem like fuffing around to some but I love my tea on the bank and having it as fresh as possible (and hot, until last knockings) is all part of the session.
I have to say that carrying two flasks as one gets older, is more of a trial as it is totally dead weight. That said, I would leave other stuff behind just to ensure good hot beverage that sees of the chill, particularly as we head into winter...Yesterday a case in point; One or two casts on a very cool morning, with hot tea in one hand and one on the rested rod, saw the float dip and slide away with a 9" roach the reward...
Let's face it, tea and kicking off with a roach - what could be better?