A more secure anchor with no locking carabiners
You’re alpine climbing. You finish your pitch, and see a nice crack which perfectly takes a couple of cams for your anchor.
(Yes, a three-piece anchor is a general standard, but in many alpine situations, two good pieces are acceptable, and you need to use what you’ve got at the end of a pitch.)
You could clip both of the racking carabiners with a 120 cm sling, tie off a figure 8 master point, and call it good. Standard textbook anchor, no problem with that.
But . . . what if you want to add a little extra security to this anchor, so you're positive the sling won't unclip?
Maybe one of the carabiners is being loaded in a funky way over an edge, and you're not happy with it. Or maybe you're a little sketched out on your two-piece anchor, and having a little extra security gives you a warm fuzzy feeling.
The main problem: you don't have any extra locking carabiners. What can you do?
Here’s one solution.
Evaluate your cam placements. In this case we’ll say the purple cam on the right is a better placement.
Step 1 - Take the racking carabiner from the better placement, and clip it opposite and opposed on the other (yellow) cam. Now you have the functional equivalent of a locker.
Step 2 - Clip your 120 cm sling to these two opposite and opposed carabiners.
Take the other end of the sling and pass it through the sewn loop on the other cam, making a basket hitch. (Don't worry about the basket hitch damaging the cam loop, it's plenty strong.)
Step 3 - Make a girth hitch master point on a large HMS carabiner. Done!
Schweeeeet! Now there's zero chance that sling will ever come undone from the cam placements. Take a deep breath and enjoy your well-earned warm fuzzy feeling. =^)
Because there are four strands going to the cam on the right, assuming ideal load distribution, that cam will see slightly more force than the one on the left. That's why we checked the placement quality when we started.
If both cams are pretty much equal quality/strength, then it doesn't really matter which cam gets the basket hitch.