This tip comes from Cody Bradford, an AMGA certified rock guide who has a ton of great climbing tips. Connect with Cody on his website.
Say you’re toproping with a few friends on two adjacent routes that share one common anchor. It can be a challenge to set this up so you have two separate master points, while minimizing anchor cluster.
There are various ways to rig this, but here’s one that’s exceptionally simple and clean. Use a quad anchor, and put two opposite and opposed carabiners on each pair of strands.
Your quad anchor could be a large length of cord (usually 7mm) tied in a big loop cordelette style, or the new school “mini quad”, typically made with a 180 cm runner, as we cover in this tip here.
To make the master points, you can use all non-lockers, all lockers, or one of each. All of these are okay; the choice depends on the gear you have available and your general level of acceptable risk.
Keep in mind at a top rope anchor, you’re not there to monitor the rigging maybe for hours at a time with perhaps many people using it. Given this, most folks use an anchor set up that is a little more secure (i.e., more lockers) than they might for a lead belay anchor, even though the potential forces involved in top roping are much less.
So, while four regular carabiners are okay, no one‘s going to fault you for the extra security if you decide use lockers all the way around. (Remember, YOU are responsible for your safety and level of acceptable risk; don't let other people's opinion or what you read on the inter-web dictate that for you.)
Here's the setup.
And here's a closeup of the anchor / rope connection.
Note that the left rope has a pair of locking carabiners, and the right rope has one locker and one regular. (You might wanna belay the person who has the car keys and owes you money on the left side. :-)