A SEP-IRA account belongs to the individual not to the plan. For the S corp to be able to make SEP contributions, the S corp must establish a SEP plan, but the SEP contributions can be deposited into the same SEP-IRA account as was used by the SEP plan of the other entity for its deposits. SEP contributions can only be employer contributions (including the self-employed as their own employer), not employee contributions (except for employee salary deferrals to a SARSEP established before 1997). To be able to make employee elective deferrals, the S corp would have to establish either a SIMPLE plan or a 401(k) plan, not a SEP plan, neither of which is permitted to make deposits to a SEP-IRA account.
See IRS Pub 560: https://www.irs.gov/pub/irs-pdf/p560.pdf
