You can walk two dogs at the same time. I think it's a good idea for them to learn how to behave. I'm sure you'd like to take them both to the same place sometime, like Petco or the park, and it would make it much easier for you. It's actually easier to start when they're puppies. At that age, you can still maintain control pretty easily.
I sometimes walk Zeus along with the neighbor's akita, Jake. The two of them combined are a little over 200 lbs. Needless to say, I'm quite outmatched physically. Now THAT made for some interesting walks in the beginning. But Zeus is well-trained, and Jake picked up from Zeus and a liberal application of treats. Now both boys walk nicely together. However, some pedestrians still look at me like I'm crazy, LOL.