Even if you could select it more than once, that doesn't really have any effect. You're not taking Dark One's Own Luck (Fortitude) or Dark One's Own Luck (Will), you're taking Dark One's Own Luck, and in using the ability you gain the bonus to one of your saves. I gather that just because you take it twice doesn't somehow mean that you can use the ability multiple times; if you performed the invocation once for Fortitude and then again for Will, it would simply switch over from Fortitude to Will.