Two Generals problem - what if we set boundary conditions, and then layer redundant messages once they've been met?
Realize first that if General $X$ doesn't receive message $X_k$, then General $X$ must assume that General $Y$ didn't receive message $Y_{k-1}$.



Suppose that General $B$ does not receive the final message $B_n$.  An important question is: Why is that?

In General $B$s mind, General $A$ didn't necessarily receive $A_{n-1}$.  And that would be bad.

Suppose in this hypothetical scenario that $B$ believes that $A$ didn't receive $A_{n-1}$.  That could be due to $B$ not receiving $B_{n-2}$.  But why didn't $B$ in this scenario receive $B_{n-2}$.  Well, possibly because $A$ didn't receive $A_{n-3}$.

That means that if $B$ doesn't receive $B_n$, $B$ must assume that $A$ assumes that $B$ assumes that $A$ didn't receive $A_{n-3}$ ... ad infinitum.

The problem here is that after $A$ and $B$ split, there is no more new common knowledge.  A nice way of modeling this situation is with the use of possible worlds.