2x6870's - and 99.9% of all other dual-slot vid cards - physically fit in PCIe slots that are two slots apart.
The problem, if any, is that an extra slot between them would allow better air flow.
Unfortunately, boards that would allow you to run 2x6870 in x16 electrical slots AND are separated by 3 or more slots are very expensive. And in most cases the only benefit you would receive is a little better air flow.
Better to put a few extra dollars into ensuring a case with good air flow and benefit your whole rig.