The key to landing the layered flicks is simple:

1. Start at the correct lane (1 for left, 2 for right)
2. Slide in a fluid motion with the same speed for both hands
3. Avoid hand or finger collision
4. Reposition back to the correct lane
5. Perform the slide again.

This gif showcases the second slide in With Love M+, but the same goes for the first set of layered flicks. The only thing that changes is the order of the flicks (First set has right hand go first, 2nd set has left) but the perfect window is so big that it's not worth the effort trying to think about this small timing difference.