I don't think that's a particularly useful example, they both describe the same incident in different words and from a different viewpoint but there is no discrepancy over what happened. Fine, that can be accepted as evidence.
However, that level of agreement is relatively rare in Napoleonic memoirs, (but these are scientists of course) more commonly you find one describing the incident in detail and the other failing to mention it and you have no way of knowing whether no 1 is inventing or exaggerating or whether no 2 either didn't regard it as worth mentioning or even deliberately suppressed it. Assuming both accounts have the same authenticity rating you can only judge by what you know of both parties and the significance of the incident to both.
Using the example you give, it might have happened that the chap who fell into the hole was too embarrassed to mention it while the other narrated it gleefully or the first might have made a big story out of a minor incident which the other didn't bother to record. You couldn't conclude from the comparison that one of them was lying but you couldn't say for certain that the incident was true without corroboration.