what 'should' happen - see page 20 (this is from the proposed regs, but this did not change in the final regs)
Designated Roth Contributions as Excess Deferrals
Even though designated Roth contributions are not excluded from income when contributed, they are treated as elective deferrals for purposes of section 402(g). Thus, to the extent total elective deferrals for the year exceed the section 402(g) limit for the year, the excess amount can be distributed by April 15th of the year following the year of the excess without adverse tax consequences. However, if such excess deferrals are not distributed by April 15th of the year following the year of the excess, these proposed regulations would provide that any distribution attributable to an excess deferral that is a designated Roth contribution is includible in gross income (with no exclusion from income for amounts attributable to basis under section 72) and is not eligible for rollover. These regulations would provide that if there are any excess deferrals that are designated Roth contributions that are not corrected prior to April 15th of the year following the excess, the first amounts distributed from the designated Roth account are treated as distributions of excess deferrals and earnings until the full amount of the those excess deferrals (and attributable earnings) are distributed.
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in other words the excess 'needs' to be segregated (or tracked or something) from the Roth account in one of the plans. and since earnings are involved you would want it separate, I'm sure.
since it was Roth, the person already paid taxes in 2017, and eventually when taking a distribution pays taxes again.
otherwise, someone could defer 24,,000 into Roth into each plan. oh I have excess but I already paid taxes in 2017. now the excess continues to collect earnings, and if not removed from the Roth account then when the person retires he gets all that tax free. a great scam, works real well in a safe harbor plan in which there is no ADP testing!
the problem, if it is in 2 plans separate plans, run by 2 different TPAs, how would anyone know? except for ethics on the part of the individual....
402A_Proposed_Regulations_Roth.pdf