Each of these rollovers is reportable on a separate Form 1099-R, one from the former employer for the rollover to the IRA and another from the IRA custodian reporting the rollover from the IRA to the solo 401(k). Each of these is reportable on the tax return for the year of the Form 1099-R, the year in which each distribution occurred.
Because a solo 401(k) does not report to the IRS receipt of rollovers, you should include an explanation statement with the tax return that reports this rollover to avoid the IRS questioning whether or not the rollover was actually completed.
