I walk them both at once. I did when I had Star and Rosie, too. They would both heel on the left side, and sit at every corner (I was so proud of them!)
Star and Sherman walk together, but I have not taught him to heel since we usually go to the dogpark so they can be off leash. Star is 8 1/2 now and she likes to walk at a calm pace, which is very nice! Been walking them at the resevoir lately, Sherman gets so excited by the sound of the waves on the rocks!