Combining with resistors should be OK. There is no need to very low TC resistors, as the voltages should be relatively close. This also makes the current flowing there small, even of just 100 Ohms are used. The main source of error would be likely possible thermal EMF. So keeping the resistors shielded from thermal gradients might help more than low TC resistors.
Not really the best idea with 732's, and this is NOT the best recommended method by Fluke to get most accurate results.
These units are pretty tough, but that doesn't mean you'll still get good results if you use them incorrectly.
Acts238 - You want to take a look at the 732a/b manual and schematic and learn what to avoid.
A) The 732 10V Hi output is source only thru the current limiter. You start paralleling these and your average 10V high will just be sitting at the highest voltage of the group (balance resistors or not) - no real averaging going on. Once that 732 10V output pin is driven at all higher than it's Vref output amp reference, the output loop basically tends to just turn off. It takes a few seconds for that loop to restabilize once the overdrive is removed; an effect usually mistaken for a noisy Vref. Head's Up: A corroded contact on 10V Hi stud on the feedback line can also cause this effect.
B) Learn why the 10V Lo pin is part of the 10V current error compensater circuit - That's on its own eco system. Whatever current 10V Hi is sourcing should be about the same returning on 10V Lo.
Notice that 10V Lo is not on the same circuit node as the other LO terminal on the front of your unit.. Both Lo returns are on same node, but 10V Lo should always be the return from 10V Hi.
You can try the cheesy way and run load resistors between 10V Hi and 10V Lo, and then balance resistors between your Hi Common and Low Common "average" rails but that adds noise and is not most accurate.
The preferred method is to make use of your low thermal switch array and cycle through your Vrefs, then get your average Vref value in data. You can also use low noise isolator amps to prevent the 732's outputs from seeing each other directly.
If you've got multiple 732's on hand then you should have multiple Low Thermal Switch banks as well - use them wisely. Your meter/s should see only one 732 Vref at a time - make sure you aren't accidentally paralleling Vrefs in your switches via sneak paths, etc.
If you're looking for noise spec, the best way to do that is to use a decent LNA filtered to .1 to 10Hz. For lowest noise your BPF filter will have a wet tantalum cap on the input to remove DC (The electrolytic cap input filter designs here on EEVblog tend to add a bit more noise, which might be important to you) and then look at that filtered noise data on your 'scope, low freq spectrum analyzer, fast DAC system, strip chart recorder, etc.
That's how we do it for best results.