Can I cascade voltage references to get better stability?
Study the data sheet.

The errors in a voltage reference include ...

a) Initial accuracy (as delivered, at 25C, with nominal load and supply)
b) Temperature coefficient (with variation from 25C, ambient and self heating)
c) Line regulation (mV of output change per volt of input)
d) Load regulation (mV of output change per mA of load change)
e) Long term stability (only specified for the higher quality references, often ppm per month)

Providing a regulated input will remove (c) from the buildup of output errors, all the others will remain. The effect of (c) would have to be much larger than the other terms for it to be worth providing a regulated input.