Depends on the make/model whether or not they require one or two power plugs. If there are two receptacles, they require two.
If you've plugged the power supply in and you can't even see a POST or get to a visible BIOS screen and these are the only two new things you're putting in your system, there's likely a problem with at least one of them. Try one card at a time. Does one work and not the other? Do they both work independently? Is neither working?