YXTM ND50 Mini grip and battery holder for Nikon D50

Does anyone know if, when two fully charged batteries are fitted into the ND50, whether both batteries power the camera or only the battery at the 'far end' of the grip?
In other words do the batteries have to be changed over when the camera indicates that battery power is low or exhausted.
Regards

Answered my own question. When two batteries are inserted the 'outer battery' is connected with spring loaded contacts that push down to allow the 'inner battery' to be inserted. If only the 'inner battery' is inserted then a slider pushes it to a firm contact with the fixed contacts. So the item allows one or two batteries to be used.

Nice to see someone else using a Nikon D50. I've two, 14 years old and both still going strong.