Was there ever an explanation as to why they're so similar to eachother, all the way down to their moveset?

General consensus seems to be that there isn't a connection, only that Shuyin's silhouette needed to look like Tidus in order for Yuna to start her sphere hunting journey.

I always liked to think that the Fayth modeled dream Zanarkand after people who existed in the real Zanarkand.