Walking two dogs at once can be a challenge, especially if you didn't train both of them to walk together while they were puppies. Not all is lost, though -- you can retrain both doggies to share leash time with you, but it will take some time, and a lot of patience. Keep in mind that walking a puppy and an older dog together might not work out that smoothly, simply because their levels of energy -- and development -- are just too different.
Train each dog separately first. You'll never be able to walk two dogs at the same time if they don't know how to walk on a leash separately. Train the dogs to walk without pulling. Practice keeping Doggie on your side, rather than allowing him to run in front of your legs. Then practice the same with the other dog.
Get short leashes for both dogs. That way you don't have to worry about the dogs getting tangled as they're learning to walk side by side. Retractable leashes would work well or simply get short rope leashes.
Grab one dog in each hand at first. Practice walking with one of them on each side of your body. This can be tricky if the dogs have different rhythms -- for example, if you have a dog who likes to walk slowly while the other has long legs and can take giant steps.
Move both leashes to one side once you and the pooches are comfortable with the situation. It might take some time for the dogs to find their joint rhythm, so they're not tripping against each other as they move forward. Once they feel comfortable -- and as long as they don't push -- get longer leashes. This will allow them more freedom to move forward and sideways.
Consider buying a "coupler" once the dogs are ready to walk alongside each other. A coupler is a Y-shaped leash with a single handle and two endings. Each ending of the Y has a snap that you can attach to the collar of one dog. Not all dogs do well with a coupler -- some dogs are too competitive or too hard-headed to adapt to the rhythm of the other dog. If you want to try a coupler, start with short walks to give the dogs time to adjust to each other and to learn how to walk together.