As far as I know, this is only possible if you're playing with a controller. Heck, it might only be possible with a PS5 controller. I did this by grappling onto two targets first and waiting for it to leave me still, as you saw, then lightly letting up on the trigger and quickly pressing it back down, which, when done right, launches an additional tether from your ODM gear. There is still a limit to how many of these additional tethers you can fire before they detach