My question would be do you really think the second block is not an illegal block below the waist? I don't see plays where there's time for a second block below the waist before the FBZ has dissolved.
I initially misread the OP as stating that the second block was above the
waist. In fact, it says that the second block is above the
knee, which might or might not be above the waist. I suspect the OP meant to say that it was above the waist.
You're right to ask whether this block is legal: if it's below the
waist, the fact that it's later than the first block or above the
knee will be irrelevant. It will be BBW, but not a chop block. If it's above the waist, then it's legal.
I also agree that there is not time for a second block before the FBZ dissolves. By philosophy, the linemen may legally BBW on their first charge after the snap. When the QB turns to hand off, the ball has left the zone, even if technically he's only 2.5 yards behind the LOS: this is a safety consideration, and we should not be looking for ways to preserve the FBZ in order to permit what would otherwise be illegal blocks.