Effect on mAh of connecting 2 batteries in series
Connecting two 1000mAh batteries in series and using them as a single battery still makes it a 1000mAh battery, but with double the voltage.

Batteries are often connected in series, but when doing so it is best that all batteries are of the same kind (brand, ...) and have the same charge when putting them in series to limit the risk of (dis)charging one (much) more than the other.