Gray Beard,
Remember, voltage is the pressure that delivers the mAh, the amount of energy stored.
Think of two buckets of water (mAh) with pumps (volts) on each one.
The voltage or pressure needs to be the same or all kinds of problems happen.
The higher voltage will be trying to push its current into the lower voltage battery.
However, the receiver will take the amount of energy it needs to run.
Therefore, the difference in the capacity doesn't matter.
It doesn't care who gives it to him, if it is available he'll take it.
If one runs out he'll take it from the other.
If the second one runs out.... we all know what happens then.
Hope this helps,
KW_Counter