They're totally different pieces.

X5 E70 left running board part #: 51 77 7 167 709
X6 E71 left running coard part #: 51 77 7 184 425

I also highly doubt that they would be compatible, even though they have the same wheelbase. The new M3 has different exhausts for both the coupe (e92) and sedan (e90), even though they look similar.

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