Hi CollieMan
Walking:
I prefer to walk one on either side, but once I get a coupler I am going to see how walking on one side works. ^^ Of course, then I got Mud...and I usually walked Nick by himself anyway, simply to avoid the rather large paws of the other dogs. ^^ Lol. But anyway, now I usually have two on one side and two on the other.
Training:
Depends on the dog. Zeke has a terribly short attention span anyway, and I can't train him with other dogs there. Not at all. So, I either take him to the field near my house or I put the other dogs either in or out while I am in/out. With Mud, I have Rusty and Zeke outside. (They like to "help," lol.) I leave Nick in though, because he doesn't really get in the way, and Mud isn't too distracted by him anymore. I prefer to train each individually in the beginning, as it seems to be just easier on them and me. Once they've got the trick, I bring in one, then later another...I think you get the idea.
I like being able to give the command "stay" and have all of them stay, of course, but when first teaching a trick I only work one at a time.
Feeding: I don't feed them side by side. My dogs aren't food aggressive, but I still just prefer not to feed them right next to each other. Mud eats in her kennel, simply because when I got her she was extremely food aggressive and her fosters had always fed her there. Plus, Rusty eats much faster than everyone else, and tends to see if anyone else has something to munch on. (All it takes is an "Aaaah!" to get him to leave their food alone, but this can sometimes lead to food aggression if you let it continue. The other dog may learn that if he doesn't eat fast or defend his food, he won't have any. So really it just depends on the dogs.)
Hope this helps!