When we have multiple dogs, we tend to think like parents and do things with them according to fairness, what we do for one we feel we must do for another. Dogs don’t worry about what is fair. So don’t NOT walk any of your dogs because you can’t walk them both at the same time and It’s not FAIR to leave one behind. Bullhockey! Take one dog today and walk the other tomorrow.
You can teach each one to walk politely on leash individually first. Then you can make it a goal, if you want to, to walk them together. I think if you asked them, they would prefer to be the only dog walked and don’t mind the every other day schedule. It gives them some alone time with their human, which they all enjoy.