Depends, if you put them in series, you double the OHMs which will reduce the output of the stereo amplifier for a given volume. If you put them in parallel it increases the output of the stereo somewhat but spreading sound across 2 speakers has some inherent efficiency losses so it will probably be about the same volume.
Speakers in parallel may overdrive your amplifier unless it is rated for the reisistance level. Home speakers are usually rated at 8 Ohms, and auto speakers at 4 Ohms. Putting speakers in parallel with the same Ohm rating drops to 1/2, so two 8 Ohm speakers will look like 4 Ohms to the amplifier.
quality car amps can take 2 Ohms, but it really heats them up.
Look at the specs of your hardware carefully, and get a meter to test your Ohms.
