I was taught an interesting way to set two pieces in opposition. This works best with pieces that are relatively close and oriented with an upward and a downward piece.
Set you pieces (one up, one down) and each gets a biner. Clip a double (or triple) sling into the lower piece. Thread the entire sling through the upper biner. Pull the sling tight and wrap the sling around the upper biner, being sure to thread the sling through the loop coming from the lower biner. Keep wrapping until you are left with the appropriate sized loop, add biner and use.
The sling is clipped to the lower piece, so it's not going anyhere. It's wrapped around the top biner enough to create a great deal of friction, and it's looped back through itself, increasing the friction, so the top piece isn't going anywhere.
A downward pull will put most of the force on the upper piece due to all of the friction. An upward pull will put most of the force on the lower piece.