I forgot the reason though, but since I feel like it was Karsten who told me before, maybe he'll fill in the blank.
Nope, no idea what you're talking about.
I can only assume that the xXx series isn't selling all that well, especially part two (is that still with the same main actor? I've never seen either). So, by selling the double pack at just slightly more money than the single you get the buyers who don't have neither yet and maybe wouldn't buy the second one on it's own.
The calculation price of the double pack consists probably of 2/3 for the second movie and 1/3 of the first movie because the first one has already dropped in value for being on the market quite a few years.