Your 2019 tax returns will report only the traditional IRA contributions, with the nondeductible amount reported on a separate Form 8606 for each of you. The amount that results on line 14 of each Form 8606 (presumably the full amount of each of your traditional IRA contributions made for 2020) will carry forward to line 2 of each of your 2020 Forms 8606.
The rollover from the traditional IRA to the 457(b) and the Roth conversion, both occurring in 2020, will be reported on your 2020 tax return. With neither of you having a year end balance in traditional IRAs at the end of 2020, the entire amount of your basis in nondeductible traditional IRA contributions from 2019 (and perhaps 2020 if you make 2020 nondeductible traditional IRA contributions before the end of 2020) will be applied on 2020 Forms 8606 to be used in calculating the nontaxable amount. As long as each individual's basis in nondeductible traditional IRA contributions is greater then or equal to the amount each converts, each Roth conversion will be entirely nontaxable. If more is converted because there have been investment gains, the gains converted will be taxable.
