Usually you write multiple .cpp files instead of multiple .ino files when splittung up code, since VSCode and other IDEs handle it better than .ino files.
Then you also have to worry about using the right code to share global variables with each other.
This has been discussed multiple times in multiple different topics, e.g., VSCode cpp: How to split source code in specific files?
Without you showing the code or the error message though, we can’t say much more.