I also have the TT and have been able to adjust the cables as discussed without problem.
Back in the days of only one cable the return spring was Strong. We used to put in a device called "Twist Assist" It was a spring that went inside the handle bar and had a bolt that extended through the grip. You could preload it to counter the return spring. Wish they were still available.
when I was in my 20's I build a suzuki gs1000 into an 1176... megacycle cams, kerker header, port / polish / big valves and springs, and 36mm cv carbs (I've always liked cv's) but the carbs only had one cable. One day this teenage kid was out in front of my house popping wheelies on a clapped out dirt bike. I thought "I'll show him how to do a wheelie" so I pulled my bike out of the shop, rode into the street, and started into a full lock turn at idle, just trying to get the bike warmed up. Well I guess the throttle cable hung on something, and the bike went to about 10,000 rpm in a split second, at full lock. It spun all the way around the front end and SLAMMED me to the ground. Took a big chunk out of my elbow... a week before bow season. I took alot of flack from that about the guys I hunted with not to be a girl about it. Right, easy for you to say

. I left on my first trip when I could pull my bow exactly 1 time. Still have the scar on my elbow from that one... and I never have another bike with just one cable - ever

Steve