Hi,
What's the problem?
Mathematics? Just subtract the values: Example: 9V - 1.5V = 7.5V
Hardware? Reverse connect the second one: example: A 9V battery and a 1.5V battery. Connect the MINUS of the 1.5V battery to the PLUS of the 9V battery. Use a voltmeter, connect the VM minus to 9V battery minus, and the VM plus to the 1.5V battery minus.
EDIT: sorry, just recognized you clearly talk about DC voltages:
Thus the following lines become invalid:
(Or are you talking about sinewave, or any other waveform?
Or RMS values?)
Klaus