Databses design, parallel many-to-many relationships. Two tables, or extra fields?
If both fields are relevant in one record (i.e. "used by" and "paid for" can both have a person_id from the persons table), a single table is a better solution. If at least one of the two fields is always NULL, splitting the table would actually make the SQL less complex.
(Also, if an expense is either "paid for" OR "used by" but not both, you could instead use an expense_type field instead of 2 separate fields).

As a sidenote, I would recommend against using space in field names.