My Dogs Fight When I Try To Walk Them With A Leash, Why Is That?

It's not that unusual for dogs that are fine with other dogs off leash, to fight when confined on leash... I'm not sure exactly of the psychology of it... But I think it has to do with them feeling threatened because their confined... Kind of like being backed into a corner.  Dogs have very precise and intricate body language that they use with one another... When that isn't possible, they have three stages of action.. Fight.. Flight.. And Avoidance.. Your job is to get them through the first two responses so that you can all enjoy the walk.  Given that they're both big dogs.. It would be best if you could initially have someone help you with the training. Find a good route, where you can keep up a steady pace without breaking.. Each of you take a dog.. Start out one in front of the other.. Once you hit a good pace, move side by side, staying relaxed and giving minor corrections as needed (they should be allowed to look at each other... You want them looking straight ahead.. Once you get them walking on either side of you avoiding one another... Have your partner hand the leash off to you without breaking stride.. Keep up a good pace, and stay relaxed, giving minor corrections if needed.  Once they understand what's expected of them... And that you are completely in charge and will settle for nothing less.. They'll fall into formation.  With two big dogs out and about... they need to understand that you are always in control.. I'd recommend you read at least one of the "Dog Whisperer" books by Cesar Millan.. I think the first one is "Becoming a Pack Leader".. They have a wealth of invaluable insight, information, and techniques to help you always stay on top... Good luck.