I think that's true. If commonly controlled, it appears the plan needs to take into account both businesses when calculating if under common control. What is a bit confusing is Diagnostics:

• The taxpayer's self-employed retirement plan deduction (Keogh, SEP, SIMPLE) was computed using the taxpayer's net self-employment income from all sources. The data indicates that the taxpayer may have self-employment income from more than one source. Net earnings from self-employment for purposes of computing the maximum SE retirement plan deduction may need to be adjusted to reflect the earnings of the business (or businesses) under which the retirement plan is established. Input fields are provided in the Schedule C, Schedule F, Partnership, S Corporation, Contracts and Straddles, Miscellaneous Income, and Adjustments to Income screens allowing you to exclude the earnings of certain activities from the SE retirement plan income base.

This may be the program saying "check to see if the Schedule C businesses are under common control as if they're not, you'll want to ignore the Schedule C's that aren't for purposes of calculating the contribution to the Solo 401(k), not sure. If anyone has insight on this, it would be appreciated.