The conversion taxation is separate for each spouse. If you each have a pre tax TIRA balance of 100k, make a 5k non deductible contribution and convert 5k, the taxable amount will be 4,760.
If you each have 200k, the taxable amount would be 4,880.
The pro rated ratio is calculated on Form 8606, which you also use to report the non deductible contributions and the conversion.
You are not limited to a conversion of 5k, only the contribution is limited. You can convert the amount that benefits you.
You are NOT converting the first 5k in your TIRA. The pro rate rules are often viewed as a "Cream in the coffee" situation. Once you make a 5k non deductible contribution (cream) it is permanently mixed with the coffee (the pre tax balance). Anything removed from the coffee is therefore a mixture of both pro rated per Form 8606.
Finally, note that if you each can roll your pre tax TIRA balance into your 401k plans (when you are eligible) you would leave the 5k of basis behind. You could then convert that 5k tax free and in later years keep making the 5k non deductible contribution and converting it tax free. Effectively, you have made a Roth IRA contribution by doing this.
There is a separate 8606 for each spouse and therefore each spouse will have their own pro rate factor. An 8606 only holds one SSN.