I tried this before and I failed, resulting in me temporarily losing the friendship of the boy. We have forgiven each other now, but should I try again?

No. You've already hurt your friendship with him once; if you try again, you're likely going to lose it again permanently. Additionally, if you're trying to break up the same couple, they're going to immediately be suspicious and catch on very quickly since they know you've tried this before. If this is the only way you can see to solve the issue that you have with the couple (whether you hate the couple, or want to be with one of them yourself), you need to take a break from interacting with them and spend time with other friends or do other activities. It isn't healthy to be so fixated on a relationship that you attempt to damage it multiple times.