How do I backup my Mac without a different storage device or computer?
Saving two copies of your files to the same drive isn’t a backup type that will protect you from a Boot Camp error.
You’ll want to get an inexpensive USB drive to copy your critical documents. Then when you can afford a drive that’s at least as large as your internal drive, set up Time Machine so you are protected by automatic backups of the Mac data. Note, Time Machine does not backup Boot Camp.