How do you A/B test two DM openings?

Use an A/B split step right after the trigger, write a different opening on each branch, then record a conversion on the outcome you care about so the two branches are comparable.

The split itself is the easy part. The mistakes are all in what comes after.

Split once, early. A split after three shared steps is not testing the opening, it is testing the opening plus whatever happened in those three steps. Put it immediately after the trigger.

Change one thing. Two openings that differ in tone, length and call to action tell you nothing about which of the three mattered.

Define the outcome before you start. Record a conversion at the point that actually matters to you: the link tap, the completed collect step, the booking. Without that, you are comparing message counts, and both branches send the same number of messages.

Let it run. Small numbers move a lot. If one branch is ahead after twenty conversations, that is noise. Resist the urge to call it early, and resist the urge to keep both branches running forever because you cannot decide.

When you do decide, delete the losing branch rather than leaving it at zero percent. A split that no longer splits is a step someone will misread later.