You mean your 17inch lcd from the alubook used as a second screen FOR the G5? This won'T work.
Or are you talking about sharing the 23inch screen between the G5 and the alubook? This works. Saw some commercials on 2 G4 powermacs sharing one screen. You might need a DVI-switch in worst case. But I think the 23inch cinema might have two DVI-in ports