When to transfer vision in 2-man

[bas2456]

I've posted a couple questions on here this season, and as a second year official, I've found it extremely helpful.

Let's say I'm the trail in 2-man system. A1 has the ball on the left wing guarded by B1, and starts a drive towards the middle of the lane.

At what point should my eyes stop watching the ball, since the play isn't technically in my primary anymore. Alot of times I find that B1 is between the Lead and A1, cutting off Lead's sightlines. Should I keep watching the play until I feel L has a clear sightline? Or is it the responsibility of the L regardless?