Can an individual/business owe more income than is earned (when operating globally)?

Suppose an individual/business is located in a country with an effective tax rate > 50%

And they’re operating in another country with an effective tax rate > 50%

And there are no tax treaties between the 2 countries… none of the taxes paid in one country can be used to deduct the tax liability in the other country. Both countries collect taxes based on residence jurisdiction.

Could there be more taxes owed than money actually earned, at least in theory? (In practice, nobody would put themselves in this situation if it actually exists in the real world.)

Yes.