Actually, it's a whole lot easier to make a sling with adjustable pouch but constant length:
Take two full-length parallel strings. Put two adjustable knots near the center. Done.
(well, you might want a finger loop and a release knot, but that's optional)
Alternately, if you don't want to hold on to a 4-strand-thick bundle while you sling, you can make one split-pouch sling any way you wish, then partially bind the ends together.
To bind the ends of a split pouch made from string, you can use whipping.
To bind the ends of a leather split pouch, put holes/rivets in the leather at regular intervals. You can then tie the ends together to reduce the size of the pouch, using wire/shoelace/anything.
What the heck, you can even sew buttons on a ribbon-like split pouch (i.e. webbing or woven)
Go wild, you could even use two zippers AS the pouch. (I don't think a zipper would slide too much, but it might get damaged)
Hope that helps