Bookshelf speakers-double up,kef ls 50

Well, they may play louder but, otherwise, it would be a mistake. Using two per channel will corrupt the excellent imaging of these speakers as the stacked pair will be very sensitive to vertical angle and vertical head position.

If you want louder and/or deeper, add a decent subwoofer.

Place them back to back and you'll get a bipole dispersion pattern. If you place them correctly you can achieve a very deep soundstage. A point source speaker like the KEF would be perfect for this type of experimentation.

Most amps will triple in power even! You could end up damaging your speakers or just playing way to much power thru them. 2 speakers connected to 1 AMP tap will make the amp see as much as 1-2 ohms! Most amps dont like this high resistance and will struggle or even shut down after a while.

Matt,
speakers in parallel will only load an amp. as long as amp has capabilities to drive them both per channel, one can easily experiment.
when CORRECT math is done, either 2 speakers connected to 1 amp or 1 will have same current going through them if amp doubles the power to the halved load or less current if the case is different.