The minimalist IRS Instructions (Schedule C) in reference to community property states where both spouses own and work in the business and the business qualifies to be treated as a disregarded entity state:
"If both spouses participate, the income and deductions are allocated to the spouses based on their distributive shares." (see https://www.irs.gov/pub/irs-pdf/i1040sc.pdf)
Just allocate the premiums on the same basis as you allocated all your other income and expenses.
"TurboTax seems to warn about the insurance premium being attributable to just one business." I am not sure where the warning is, but I suspect that it is in reference to a taxpayer owning more than one business, not a disregarded entity co-owned by two spouses. After all, you really have only one business, which is being reported artificially so that both spouses get their due credit for Social Security and Medicare taxes (Sch SE). Otherwise, one spouse would be working but not getting any SS credits for retirement.
