Don't immediately rescue a freed human once you've picked him up. Instead, wait for another human to become free, then start shooting the new one. This bounces the human into the air. You can juggle the human to the rescue point, then boost through it to save the one you're carrying.
An alternative strategy is to set the first human down next to the second and juggle both of them with your fire at the same time.
This should probably be done on Acis, as you won't have to worry about missing a juggle while fighting off enemies and having the human fall off the map/into an environment hazard.
Edit: I've included LilGoody2Shoes's Youtube video for reference, as well as rdfa81's alternative strategy as seen in the comments.