Yes, with only the solo 401(k), your wife can just use the Maximize function for an individual 401(k) to determine her own maximum individual 401(k) contributions.
TurboTax just implements the Deduction Worksheet for Self-Employed in Chapter 5 of IRS Pub 560 as TurboTax's Keogh, SEP and SIMPLE Contribution Worksheet, so you can review the calculation there (taking into account that you can't make an elective deferral to your own solo 401(k): https://www.irs.gov/pub/irs-pdf/p560.pdf
