I've been given the advice before that two main characters starting with the same letter can get confusing. Is SH and CH fine, since they look different, or is the pronunciation a problem?

in my humble opinion, they both sound very fine, and I see no reason why they could become confusing ... I'd keep both names ... :) ... good luck to you evil

That shouldn't be a problem, especially if they are vastly different. Shane is usually a boy's name, but there have been a few female Shanes. I think the bigger problem are names that are completely foreign, like ancient Goth or Norse names - Raedwulf, Raedward, Grasulf, Grothgar, Aeldward.... There is a trope called "There can be only Steve"; just don't have a Steve, Stevie, Stephen, Steven, Stephanie, lol.

Replying to justinc.06t: "There can be only Steve"; just don't have a Steve, Stevie, Stephen, Steven, Stephanie, lol. ↩️ I feel like that rule should be applied to regular life, as well. I once had a class with a Chris, Christine, Christie, and Crystal 😭

I used to work at a company with a LOT of Brians/Bryans, Michaels, and Johns/ Jons. Eventually, everyone had to use last names and nicknames. The Brians became B-[last name] like B-Chan, B-Kim, B-Lars (Larson), etc. That became part of the company culture.