Acting mods are becoming hard to find for later versions of Minecraft and I'm trying to find alternative ways to create machinimas. Thankfully this mod is still supported and has good camera functionalities. I was wondering if there would be any way to overlay multiple replays as to have different characters in the same scene? I know this mod's purpose is to record gaming sessions, but if there was any way I could still use this, I would be grateful.

This is out of scope for ReplayMod itself. But a tool for editing replay files (called Bob) is in development by Rader, one of the ReplayMod Discord's moderators.
The approach is a generic, scriptable tool. So extracting data and injecting it to another file should be possible with that.

Find the ReplayMod Discord at https://discord.gg/5GR7RSb
And the Bob's Discord at https://discord.com/invite/zNVkeGdjKt