I tried using double cable clamps like that when I first built my car and they tend to let the two cables slip past themselves and stretch the overall cable out - not good. Once I went to the cable end studs shown above I never had any more troubles with them slipping when pulled.
I doubt that the cables have stretched that much - unless there was a Gorilla installing them in the first place - Those are 3/16" aircraft-style cable and I seriously doubt that pulling on the handbrake handle is gonna stretch them unless you stand up on the seats and pull with both hands like crazy. Pull the cables out of the end studs, maybe, but stretch them that much? Nope.
Trim them back a little bit and try re-adjusting them. You can grab the cable end with some vise grips to pull the slack out (just remove slack - don't try to pull hard enough to begin to engage the e-brake shoes out back), then eyeball where the end stud should be (don't forget to allow for adjustment). Draw a line on the cable at that point where the cable will "bottom out" in the stud barrel (or a tad longer if you're not sure) then wrap some painter's tape or electrical tape around the cable where you want to cut it so that the cut will be in the middle of the tape, then use a Dremel with a fine cut-off wheel to cut the cable. Doing that will prevent fraying of the ends so the cable will go into the end stud easier and the set screw will bite better.
When you adjust the handle, you want to go for three or four clicks up from fully down. 5 - 6 clicks are OK, but that's beginning to get "up there" (remember that future adjustment thing).