Splitting is easy.
Just add all your files to the TOC. (This requires a restart of the WoW client if changed!)
Example:
https://github.com/zorker/rothui/blo...oUF_Simple.toc
In every addon file you have access to globals, locals and your addon namespace.
Lua Code:
--addon name, addon namespace
local an, ans = ...
You can add functions and objects to your addon namespace without poluting the global namespace.
Making an object global, keeping it local or adding it to the addon namespace is up to you.
Example:
https://github.com/zorker/rothui/blo.../functions.lua