I'm not sure the difference between an SSD and HDD will be very significant compared to the power the rest of the system draws.
It depends on how much power the system draws, and how efficient the SSD and HDD's are as they vary quite a bit too.
Some laptops may draw 15W, and some 50W so it's hard to guess.
UPDATE: HDD's can also be set to go into an IDLE state in power savings (after say 20 minutes) and there are other more significant power savings such as adjusting the backlight strength (brightness). See power options.
Most laptops don't have support for two drives either.
It might, but drives are hardly what causes significant battery drain (other than the 960 evo/pro, and they don't even suck power until you install their drivers). Having 2 SSDs instead of an SSD/HDD combo will barely make a difference in battery life at all.