well it depends on the type of amp circuit it is. sometimes you can bridge the outputs of a stereo amp to a single speaker.
you can either feed the mono input to both l and r and use two speakers.
you can use L and R as a 2 input amp each channel has their own speaker or if its bridgeable into 1 speaker. or you can just use 1 side and 1 speaker and use it as a mono amp. the unused channel will still use a little bit of power so it might not run quite as long from a battery.
You can use a single/mono input to 2 amp circuits for stereo output or a mono input to a amp that has stereo output.
The output wattage is listed as double, but in reality it's 2 circuits giving the same output that's only half of the listed output. Example: a 20 watt stereo amp is basically 2 10watt channels leading to their own speaker.
Stereo amps need 2 speakers to be stereo. If both outputs are wired to 1 speaker, you could run into issues with overload to the amp circuit or speaker coil depending on parameters.
What amp/amp circuits are we speaking of? What are the amp/amps specs? What are the speaker specs?
The amp is this one:
The intended speaker is an old Kicker 3.5" 2 way, rated at 40 watts.
I tried one of these using two little computer speakers in a cigar box. It has a mono jack into both channels. Pretty lo-fi but it was only a couple bucks. You won't get much out of it without a preamp, I just use a pedal.
That amp is a 2 channel amp rated 15watt max at 4-8ohms per channel for a combined 30watts for both. It's just 2 15watt channels and not a true 30watt.
Use 2 speakers. If you wire both channels into one speaker, it will likely damage the speaker coil due to overloading the ohm impedance. Just one speaker on one channel of a stereo amp will leave out some of the sound range if it's true stereo and not just 2 mono amps paired together.
So try the speaker on each channel to see if there's a difference on either one. Use the speaker on the one that works best til you can get another speaker. If it's just 2 mono paired, it won't matter which side.