Sounds like I will just have to experiment and play it by ear, so to speak.
The cats encountering something during their walks that spooks and/or excites them is what I'm most concerned with. It could be as simple as seeing a squirrel, or someone else walking their dog. Even with no external trigger, I can envision both cats wrestling with each other, then feeling the need to bolt off in opposite directions. I can deal with one cat in distress... two, that would definitely be tricky.
And I do realize that a cat can get out of its harness if it really wanted to. They key is exercising proper leash control
which I've learned (sometimes the hard way) with Newt. I imagine leash control with two cats facing different directions would be close to impossible.