Yeah, a predictable trigger is essential, because you're trying to get the second shot to break at a specific point in time. Kinda like trying to shoot skeet with stationary fixed point aiming. But instead of waiting for the clay to move into position, you're waiting for your sights to line up on the target.
If the second shot doesn't break exactly at that point, your sight picture tends to wobble a bit before it settles down again, greatly increasing your split time. The tempo of the doubletap will vary between guns and individual people based on their grip.
After finding your stride with controlled pairs, you can start hammering (same tempo, but you can keep your focus on the target instead of the sights), and your shot groups will stay tight.