Correct. In your example, you would have a total of $20,000 in traditional IRAs of which $6,000 is basis in nondeductible traditional IRA contributions. A Roth conversion of $6,000 would result in $4,200 being taxable and $1,800 being nontaxable. Afterwards you would have $14,000 in your traditional IRAs of which $4,200 would be basis in nondeductible traditional IRA contributions.
Form 8606 is an individual form, not a joint form even when included with a joint tax return. Each individual has separate IRA totals and basis. If both individuals make nondeductible contributions or do Roth conversions, your joint tax return would include two Forms 8606, one for each spouse.
