if you connected two 1s 650 mah lipo batteries in series to form a 2s lipo.

if you connected two 1s 650 mah lipo batteries in series to form a 2s lipo. What would be the amh of the 2s battery?

The capacity is 650 mAh regardless of the number of cells in series. If cells are connected in parallel, capacities add.

Connect in series voltage adds, capacity (mAh) is unchanged.
Connect in parallel capacity adds, voltage is unchanged.

Steve

But you now need a charger which can balance charge the series arrangement.