The regular 401(k) addition limit is $69,000 limit is per employer. The $7,500 catch-up contribution limit is an individual's limit which can be split between the two employers to be applied on top of the regular elective deferral limit of $23,000. Absent any after-tax employee contributions, your maximum permissible combined contributions would be accomplished if you make the maximum elective deferrals, including the maximum catch-up, to the corporate 401(k) plan, with the additions limit at that employer being $76,500, and make the maximum permissible employer contribution to the solo 401(k) with the additions limit to the solo 401(k) being $69,000. Of course to be eligible to make a $69,000 employer contribution you would need $345,000 of net earnings. Net earnings from self employment are net profit minus the deductible portion of self-employment taxes. Plan permitting, after-tax contributions could be made to bring the total additions up the the $76,500 and $69,000 maximums.
(I'm also assuming that you have no significant ownership or control interest in the corporation that would require the two businesses to be treat as a controlled group which would require the two businesses to be treated as a single employer for the purpose of a retirement plan which would preclude establishing a solo 401(k).)
