The fans would be placed in parallel, so yes they will see the same voltage.
However, there are no guarantees that your motherboard will be able to deliver enough current to drive both fans. The voltage could sag (so that the two fans would not see the same voltage that a single fan would), or the motherboard could detect an overcurrent situation and do something about it (potentially refusing to boot, or just not turning the fans on at all).
Your video card(s) doesn't have a fan header?