I bred, own and hunt three field trial Labradors. I always have two in my truck when hunting waterfowl or upland birds. When the oldest grandma could still hunt (she's 13 now), I would have three in the truck. On long hard days, it is optimal for me to change out dogs, and I can pick and choose easier hunts for the younger dogs to participate in where they will get the most out of it with the least negatives or potential problems arising until they totally learn the hunting game, which is very different than running a trial dog. The older more experienced dogs get the really nasty pheasant cover or the goose hunt where the guys will likely be shooting a lot of big birds. And I will run multiple dogs at a time in wide open spaces such as sharptail hunting. Fun to watch a couple of black bounding blobs in native prairie grass chasing chickens together...
No down sides to owning and running more than one dog. Except the $$.