Turbo Tax is doing it right. Why do you think it is wrong?
This is a common misunderstanding. If you are both over 65 and in the phaseout amount, the 6% reduction is subtracted from EACH spouse's 6,000 effectively making it 12%.
It is phased out for EACH spouse. Fill out Sch 1-A yourself and see. It's pretty easy. Then why would they even have 2 lines 36a and 36b for spouse on Schedule 1-A? And the max for Joint is 250,000 and not double for Single (350,000).
IRS Schedule 1-A
https://www.irs.gov/pub/irs-pdf/f1040s1a.pdf
See IRS instructions for 1040 line 110
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
