SEP contributions are employer contributions. Unless the single-member LLC and the corporation are a controlled group that must be treated as a single employer, these are separate employers and must have separate SEP plans. Income that you receive from the corporation cannot be combined with the net income from your single-member LLC to make a contribution to the SEP plan that covers only your single-member LLC. However, although probably uncommon, separate companies can make the SEP contributions with respect to a particular individual under the separate plans of each of these companies to the same SEP IRA account of that individual since it's the individual who owns the account.
If the details of the ownership dictate that these two companies are a controlled group, such as would be the case if you owned more than 50% of the corporation (including shares owned by family members that must be included due to attribution rules), the single SEP plan must cover all employees and shareholders of both the single-member LLC and the corporation. Any SEP contribution made under a SEP plan must be the same percentage-of-income contribution (up the the maximum income considered for this calculation) for all eligible employees and shareholders (of both companies) covered by the plan.
