This is a great question!
I have two dogs as well, one is a puppy, and one who is a sled dog and therefore has the endurance of...well...a sled dog. If I just let them loose the puppy, Apollo, would exhaust himself in the first couple of miles and be done before we even got started. Hadley, the sled dog, not only has the stamina that comes with experience (and the breed) but also knows when she needs to slow down for the long haul. Apollo doesn't have the experience yet to understand how to pace himself.
As such, and also because we have recently moved from Alaska to Washington, I almost always try to keep them both on leash when we're out on the trail. When we lived in Alaska I rarely had to worry about keeping Hadley on leash because we were in the interior and traveling over tundra and land that had minimal brush and were often above tree line. Most trail regulations required that your dog simply had to be under voice command, which Hadley is really good at.
Now that I'm traveling with two dogs, I make sure to keep them on leash as often as possible. For one, out of respect for the other folks on trail, but also for the safety of the dogs, particularly because Apollo has minimal experience given his age. The likelihood of one of them getting injured bounding through the woods is significantly higher now that there are two of them (and also because Apollo believes he can do anything Handley can do). Additionally, if we're doing big miles on a trail, we will go further with Apollo on leash and sticking close that we will if he burns himself out in the first mile of trail. Either way Hadley will get a workout and be tired after a big day too.
When we're walking, whether on trail or around the neighborhood, we use the Ruffwear Trail Runner System and a pair of Ruffwear Roamer Dog Leashes. When we're on longer trips we'll attach the leashed directly to the hip belt of our packs. When Apollo gets a little bit better walking alongside Hadley, we'll likely get one of those split leashes that allows you two have two dogs on one leash. We'd end up as a horrible wound up knot of leashes, dogs, and humans if we tried that method now!
Hopefully this helps a little bit, thanks!