How much can I push the limits of a device's VDD?
Sure, you can use single supply for both, if you are confident that the supply voltage right at the chips will be withing operating conditions of both chips at all times.
This includes calculating the ripple+noise, load regulation, and any voltage drop over PCB track resistance and filtering ferrite beads that may exist on the supply path, under all input voltage and output current conditions your design will encounter.