Leone's original opening idea for OUATITW of having.. Eastwood, Cleef & Wallach in dusters to meet Harmonica at the train station with the conclusion of all three of them dead on the platform, would have explained this idea alot more clearer I think.
Maybe they're both well known in their profession as con men/murderers? I'm pretty sure if they're as professional as they are at their illegitimate careers that they must be known in the 'underground' world. Remember in For a Few Dollars More how Mortimer finds out about Manco by hearing about him from the man giving him the reward money + the news paper article. That's probably how Leone's world works I'm guessing.I kind of think that's how it is with the whole Harmonica/Frank thing too. Maybe Harmonica just knows a lot about Frank because he's well known in their world? Harmonica might've even known who Frank was before he was on his tail. "Faking evidence is one of Frank's tricks" he probably caught up to him and found out information from those who had issues with him.There's quite a few Leone characters that are "well known" because of their reputation. Ramon and Indio being two of them(One of them, even!). Sorry to have gone a little off-topic, but those two issues are kind of related. The answer to all this talk about "How does so-and-so know so-and-so!?" might be because it's just their reputation and since that might the case it would seem redundant for Leone to constantly repeat how characters are well known to other characters, yet we've kind of had a glimpse at why and how. Ramon by word-of-mouth, Manco by word of mouth and newspaper articles, and maybe that's all we need to know to understand how that system works. I think that shows Leone's genius, introducing the way his characters work and when you understand it he lets it go, so we know how it is but we don't have to be told over and over again
For one thing buggers me a lot, I have realized that that Douglas Mortimer, brilliantly interpreted by Lee Van Cleef isnt quite the violent and cruel Sentenza from The Good the Bad and the Ugly.