Sorry if this topic is closed. I just started hacking myself and this was one thing I just attempted for personal use. Here’s one that someone else did [FE8] Sacred Stones - Route Fusion
If using Fe builder, there are 3 changes you have to make for a basic route fusion. Change the next nodes on the world map node editor, for Ephraim 1 and Eirika 1. Change the goto next chapter in the end event script of each chapter. Remove duplicate recruitment events.
It’s much easier if u just dictate which route to use as base, but u can also allow either route to be selected and do conditional events based on the game mode.
Besides these you’d wanna force split the party and edit the cutscenes for the chars on each route.