That's a pairing I haven't seen a lot of, honestly, so your options will feel a bit scattered. My main tip is to rely heavily on the archive tags, because 'Hermione x Ginny' by itself is mostly going to be Hogwarts-era stuff from 'Harry Potter'. If you want cross-universe, you need to look for crossovers. On Archive of Our Own, try the relationship tag for 'Hermione Granger/Ginny Weasley', then filter by the 'Crossover' tag. You can also exclude the fandom 'Harry Potter - J. K. Rowling' to force it to show you stories where they're placed in other worlds, but that's a gamble—sometimes the tagging is a mess and you'll miss good fics.
For a more targeted search, look for crossovers with fandoms you already like. Say you're into 'The Legend of Zelda' or 'The Witcher'; search for those fandoms plus the Hermione/Ginny relationship tag. It's a lot of sifting. I once found a weirdly compelling one where they were both pilots in a 'Star Wars' AU, but the author orphaned it, so I can't even find their other work.
The other thing is, this feels like a ship that thrives in smaller, prompt-based communities on Tumblr or Dreamwidth. You might have better luck searching a phrase like 'Hermione and Ginny dimension travel' on Tumblr, though the format will be snippets and headcanons rather than full stories. It's niche enough that the big platforms don't have a ton, so you really have to dig.