If neither participates in or owns any part of the other's business, the attribution rules that would otherwise result in these businesses being treated as a controlled group that would have to be treated as a single employer for the purpose of a retirement plan do not apply. Each can set up a separate SEP plan (and, cannot set up a combined SEP plan).
The maximum contribution is 25% of wages paid (but not to exceed the maximum limit for the year, $57,000 for 2020 which would require at least $228,000 of wages paid). Your pass-through income is not wages and does not factor into the calculation of your permissible SEP contribution (although the SEP contribution made by your S corp and deducted on the S corp's tax return does reduce the pass-through income). Catch-up contributions are not permitted in a SEP plan. SEP contributions are employer contributions, not employee contributions.
