The voltage can be whatever you want, but understand that less voltage means less transconductance or more capacitance, give or take exact specifics of the transistors in question. On the other hand, lower voltage means higher current for the same power limit, if operation is limited by power dissipation. That can be helpful.
Transconductance is usually ~flat until saturation, and the bottom transistor sees little change in voltage thanks to being constrained by the top one; so it can have low headroom, and fairly high capacitance, without significant impact on operation. This in turn leaves more voltage swing for the top transistor, and it will have lower [average] capacitance over that range.
A dramatic example of this was from some CRT monitors I once salvaged: bottom transistor PN2369, a fairly large (200mA), low voltage (15V), reasonably fast type; followed by a BFQ225 (100V 100mA TO-202 5W 1GHz) to deliver the required swing (>30Vpk at >50MHz).
Tim