Came across this chart showing sales of X6 vs. Z4 and 6-series.
The dark brown/gray line shows X6 sales, blue line shows Z4 sales and yellow line shows 6-series sales.
For the most part X6 sales are close to Z4 and 6-series, since the X6 entry in 2008. Some deviations due to Z4 and 6-series MY exit/entry. Also, Z4 sales are seasonal.
Source:
BmwConfig.com | Historical sales of BMW vehicles