Monitor the 10V instead, because that range / value is the most stable on both instruments
Set 3458A to NPLC 50 .. 100, AZ on. Make an ACAL after 6h warm-up period.
Place the whole setup in a stable temperature environment, i.e. +/- 0.2.. 0.5°C at most over 24h
Use a monitoring program, which is able to frequently monitor (e.g. each 100 samples) the 3458As internal temperature TEMP?
Monitor over 24h, date, time, 732A value and TEMP?
That way you can get an idea of the initial stabilization of the 732A, its 24h stability (if that of the 3458A is already known, and its internal T.C.), and the common noise.
Plot 732A value and TEMP? over time.
See examples on my 24h stability measurements on references in the LTZ1000 Super Duper Thread.
Frank