Before you spend time figuring out how to calibrate your voltmeters, figure out why you would want to. Those requirements will tell you how well you need to calibrate your voltmeters, and also tell you whether those two will ever meet your needs.
If you "just want it to be right" that is fine too, but it is the first step to volt nuttery, which can be a long and expensive habit.
I spent an hour or so yesterday figuring out a calibration sequence for my test gear so as to use the two pieces of gear with a current calibration and a GPSDO and 40 pS rise time pulser to calibrate the rest of the gear.
It's been 30+ years, but IIRC the Micronta DMM I have one pot adjusted the voltage and the other adjusted the resistance. The LCD is dead so hard for me to check that. But as long as you mark the location *before* you attempt to adjust it, you can sort it out.
You should read Conrad Hoffman's "Mini Metrology Lab" series. He's a regular here, so he should show up at any time. Fresh alkaline cell calibration is free if you buy cells you use. The $3 AD584JH is a very good deal and gives you 2.5, 5, 7.5 and 10 V references. I bought 3 to do a study on aging behavior.
There are several forum members who could probably buy a house with the proceeds of selling their test gear. So the risks associated with becoming a voltnut are quite serious.
You might also want to look at:
https://www.eevblog.com/forum/testgear/habor-freight-giveaway-dmm-test/Cheap meters are quite accurate out of the box, and with a little TLC even better. Sadly the last one I got does not have the trim pot. But the manual tells you to calibrate it annually. :-(
Fro background, I have two 6.5 digit HP 34401A DMMs. Both are in spec relative to my DMMCheck Plus which was calibrated before shipment against an in cal HP 3458A. One 34401A had never been calibrated since it left the factory.