Two multimeters, two different voltages. Now what?

[J-R]

Quick side note, for US buyers there is also the PentaRef from VoltageStandard: https://voltagestandard.com/pentaref
They are out of stock right now, but he has more coming soon.
You can choose any five voltages at purchase to closely match the counts on your DMMs.  So maybe something like 1V, 1.9V, 5V, 6V, 10V would cover most common handheld DMMs?  Some 50k count DMMs might be 55,000 counts or 49,999 counts, so it could depend on the calibration point that you're trying to reach.

[khs]

As a poor man's start drug I would buy me three MAX876BESA+, 10V ±0,05% for about 6 bucks each.

[iMo]

Some time ago they carried a dozen of clocks in order to navigate their ships..

[CatalinaWOW]

Some time ago they carried a dozen of clocks in order to navigate their ships..

We do the same today even at the cost is hardly a concern end of voltage evaluation.  Arrays of Josephson junctions.

[J-R]

FYI, the PentaRef with enclosure is back in stock: https://voltagestandard.com/shop/ols/products/pentaref/v/PentaRef-add%20enclosure

[Fluxanode]

Get yourself a new FLUKE meter.   

[Monkeh]

Get yourself a new FLUKE meter.

And now you have a third unproven reading. And a credit card bill.

[CalibrationGuy]

I think it was already said but take the best one and calibrate it (with data) and use it as your reference.

TomG.