I'm not sure it's possible to double-gather food from the same tile in a single turn, but if you could I think that would widely be considered an exploit. (There are intra-turn exploits possible, but I think mainly you get the benefit of high food tiles and high prod tiles, not double of any given FOP.)
But I often share high-food tiles between towns and alternate which town gets the food bonus in any given turn. For a simple example, if one town has 3fpt surplus, then one out of every 7 turns (without granary) it can let another town borrow that tile and both towns grow faster. 3fpt with granary you can share the tile out 2 out of 4 turns. It can make a difference, especially in the early game.
Naturally the shared tile has to be within each town's workable tiles.