@FLouise , most SEP plans use Form 5305-SEP for their plan agreement. If your SEP plan is based on Form 5305-SEP, this form that you signed indicates that you are not permitted to maintain another qualified retirement plan such as the solo 401(k). The maximum employer contribution to a SEP plan is calculated the same way as the maximum solo 401(k) employer contribution, so there is generally no reason to have a SEP plan when you have a solo 401(k).
As far as I know, the main reason that Form 5305-SEP does not permit other plans to be maintained is for exactly the reason for which you have a concern; Form 5305-SEP does not provide any instructions for determining the maximum permissible SEP contribution when you have multiple plans.
If your SEP plan is based on Form 5305-SEP and given that you've violated its requirement that the SEP be the only plan, I would think that the SEP plan is invalid and that entire SEP contribution is an excess contribution. If your business maintains the solo 401(k) for 2022, you would not be able to apply the excess SEP contribution as a SEP contribution for 2022. In this case, see the IRS fix-it guide:
https://www.irs.gov/retirement-plans/sep-fix-it-guide-contributions-to-the-sep-ira-exceeded-the-maxi...
If your SEP plan instead uses a prototype SEP agreement other than Form 5305-SEP and allows the business to maintain both a SEP plan and the solo 401(k) for the same year, if the employer contribution to the SEP plan was made in 2022, any portion of that contribution can be treated as part of your SEP contribution for 2022 rather than part of the contribution for 2021.
