I have a '79 with double cables.
I get the bottom nipple in place on the puller, and then the top.
Then I put the narrow end of the cap in place to capture the top nipple, and pull it backward (toward the grip) until it drops in place.
edit: I'm not sure, but I think I have to grasp both cables with thumb & forefinger, and pull the sheaths back at the same time. This has the added advantage of putting tension on the inner cables and keeping the nipples in place.
I'll admit it took a few times before it became easy, but that was so long ago...

I find this procedure about 1000% easier than the drive shaft boot installation.