Copying a Gmail account

Has anyone been able to successfully copy the contents of their gmail account to a second gmail account?

I’ve tried this method several times and it never works properly (one time it got almost all emails, one time it got maybe a day’s worth, one time it got maybe a thousand random ones, etc.)

I’m right at the 15GB limit and would much rather just split the account in half than spend days carefully reading and deleting stuff.