I am making a game where I need the character to perform two animations at the same time - one for the lower body - i.e. legs and lower torso, and another one for the upper body.
My question is if it is better for me to just write a new custom animation script for my game or try to use the one provided by Roblox?
I’d suggest you splitting these two animations into one.
You’re referring to the default animation editor, right? You can still use Roblox’s anim script, I don’t think that justifies rewriting a new one.
If your animation data is such that specific body parts aren’t included upon export, those parts remain unaffected when you play that animation.