Hello timoteo1,

May be 2 ways...
With standard Camera, try this:

-1-
Manuel, page 299... Add a twin key frame for your A and one other twin for your B.

OR

-2-
Slide your A few out of the time line on the left. Slide your B out of the end of the animation, on the right. And edit the filter very straight (without smooth) to have the duration like you want.

;-)

Philippe