No matter how tight the linkage, there will always be a little slop in there.
At idle, the throttle plates/shafts rest on the idle speed adjustment screws.
As soon as you move the throttles off their stops, the linkage pulls on each throttle(through the cable pull). It is imperative that the actual cable or cable attachment point does the pulling. This replicates what happens when you are driving.
I find it best to have a helper hold the throttle steady, but if that's not possible push/pull from the cable attachment point. Surely a friend or spouse is available for ten minutes or so? Gordon's tool is fine, but I'd like it better if the pull was exactly from the cable. Anywhere else can induce some twist.
Like I said above, 3k rpm isn't necessary, in fact it's better down at 1500-2000 rpm. The smaller the throttle opening they are synched at the better it will be. As the throttle opens synch becomes less and less important. Think about it, at half to full throttle any minor difference is not noticeable. But at a very small opening, any difference in airflow will be noticeable.
Getting it right on makes all the difference. When it's perfect it drives so smooth it can feel like a different car. That is why any and all linkage slop or wear must be eliminated.
Linkage synchronization has nothing to do with which jets are in play, it is ONLY about equal airflow.