In many protest involving rule 18.2 of the Racing Rules of Sailing the crucial fact revolves around whether an overlap was obtained or broken in time. What is allowed within the rules, after the first boat enters the two-length zone, depends on that. If the sailors disagree and there are no witnesses, who were in a good enough position to see, there will be doubt. The protest committee will then ask more questions to resolve that doubt, or conclude that this doubt is reasonable.
In the latter case the protest committee will apply rule 18.2(e) to resolve the protest. They go back a couple of boat lengths - as far as needed - and ask again how the overlap situation was. Somewhere in the leg leading up to the mark rounding, there will be a point where both boats agree on a overlap position.
For example, there was an overlap at that position. After that, one sailor claims the overlap was broken and the other says it wasn't. Rule 18.2(e) states: it SHALL be presumed that she did not - in this case - break the overlap. The protest committee has no choice and must follow this rule. If the starting position, which both boats agree on, is one without an overlap and after that one sailor claims he did establish one and the other disagrees, the same is true. The PC must find that there was no overlap at the two-length zone. Once reasonable doubt has been established, they have no other way to go.
Some of the sailors, who have been in such a protest-hearing, are very disappointed that their word is not good enough, that they are not - in their eyes - believed. They forget that committee members have not seen the rounding themselves and must judge on the stories of both sailors. Instead of relying solely on their own observations, they should apply the principle of rule 18.2(e) on the water. If there's reasonable doubt, i.e. when the other boat disagrees with your assessment of the overlap situation, assume that the PC will also have that doubt, and handle accordingly.