You say your client has 20% SSTB, so that means he cannot split. the irs says 10%. Just want to point out The IRS also says,  The SSTB label is irrelevant if your 2023 total taxable income (which includes non-business as well as business income) is $182,100 (or $364,200 if you're filing jointly) or less. At these lower income levels, owners of SSTB as well as non-SSTB businesses can qualify for the same QBI deduction. 

The QBI deduction phases out for SSTBs. You'll still qualify for a partial deduction based on the applicable percentage if your taxable income before the QBI deduction is:

• More than $182,100 but less than $232,101 if single, head of household, married filing separately or qualifying surviving spouse, or
• More than $364,200 but less than $464,201 if married filing jointly