The S-Corp can only contribute to the S-Corp's plan, the SEP plan.
If the S-Corp's share of the partnership was 80% or more, which seems unlikely, I believe that these would be a controlled group required to be treated as a single employer for the purpose of a retirement plan and only one plan would be permitted, which would presumably the 401(k) plan since the 401(k) plan permits both employee and employer contributions.
