Can I make my stereo play mono by...

Amplifiers are high current, and they have an idea of what the signal should be.

If 2 amplifier channels are summed by a cable, they will usually fight each other, providing their maximum output current to win.  Imagine there's a trumpet on the left. but not on the right. The two amplifiers will move all their current to change the voltage to what they think is the correct signal. One will fight for the trumpet, the other will fight for silence.

That is why you can't just use a wire to connect the outputs.

... thank you for your explainations - in this case I guess Electrical Engineering trumps common sense...

Leave your pair of speakers in their normal location. Use the "mono" function on your preamp (if it has one). You will have a nicely centered mono image between the pair of speakers! Leave the amp connected as usual for stereo.

I find that using a pair of speakers this way for mono is much better than a single speaker.

Only two-channel tube amps can be parallel-connected! This will give double the power for mono use. Never parallel the outputs of a two-channel transistor amp! Some of these amps have internal bridging circuits that will allow conversion to mono safely.