Depend on the mobo you have, its true that some multi GPU mobo can only give 8x when slied.
But you just have to look at the spec and buy a mobo that have 2 16x slot.
My Old P5N32-e SLI PLUS is a dual 16x mobo. If my old mobo is ! I think lotta new one are
And yes, your X2 card is recognized by the MOBO as one card, so YES its going to be 16x for your 4870X2.