It's been years since I have taken two cats out on a leash at the same time. The first two or three times was just around the yard at the house, and there was no problems. The last time I tried it was a fiasco. One of the cats walked behind an old broken hockey stick and it fell over hitting the other cat putting her in the fight or flight mode. She took off on the dead tear and came to the end of the leash about 4 feet away from me, at this point she turned and looking for someplace to go decided to run up me. Fortunately I was able to stop her before she got to my head. But by the time I pried her off my chest she had embedded three of her nails deeply into the breastbone. I had to pull her nails out of my chest after I got her back on the ground and settled down. At that time I had no idea how strong a cat could be in a state of panic. The other thing was they hadn't had enough training before I had tried taking them out on a leash together.
I think if I were you I would make sure Newton was well-trained to your satisfaction, and if I were taking them out together I would have someone with me, who the two cats were familiar with, a few times before doing it alone. Just in case something happens to scare them severely, and you could end up in a similar situation as I had.
As far as the coupler double – leash goes I would be awful careful with it. It might work well for dogs, but cats are of different temperament and if it came to fight or flight they wouldn't be able to be separated fast enough, and that could put an end to the relationship between the two of them.
Hopefully somebody with more experience will give you some of their ideas. But that's just my thoughts I would hate to see anything come between Newt and Newton they are a great-looking pair.
It would be a lot of fun to be able to walk a couple of cats on a leash at the same time. If you decide to work on the idea I would be awfully interested in knowing the outcome,