No it will not, one of the batteries is used to turn the machine on and that is the one that dies first. I have a pair of IIfx and a pair of IIx machines so I know.

Make sure you put the battery in correctly (+ and -) is marked because it will not work unless the batteries are good and in the correct polarity. One of my IIfx was sold to me cheap because the owner replaced both batteries and put one in backwards and it didnt start so he thaught it was dead.