Yes, there is a very simple reason to do just that: it’s the case when you wan’t to make a game that works on both PC, or Console as well as Smartphones. I.e: Genshin Impact runs on PC, PS4, PS5 and smartphones. Although I don’t know which engine they used.
Unreal is more suitable for desktop and modern consoles, but the game size will be too huge for smartphones. But unity’s game size and performance tends to be much more suitable for smartphones and tablets. But it lacks high end graphics and performance of Unreal on modern big systems.
I.e look at “Underworld Ascendant” game. They tried too make a high end desktop game with unity, and it ended up a huge mess. The loading times are horrendous even on modern gaming PCs and the initial release was filled to the brim with bugs the size of a house. It was quite a promissing game before the release and made a huge splash in the ads before it was released. But after release …
Maybe it was the developers being too inexperienced. But I think choosing the wrong tool for the job was also to blame.
As for the process it’s also quite simple:
You make assets in your team’s favorite software: (Autodesk Maya, 3Ds Max, Blender, or Cinema 4D for 3D and animation or Photoshop, Gimp Krita for 2d art pieces ).
And then keep a reference version which you could export from that to any engine you like.
Hope this helps