At the risk of sounding like an adult diaper, it depends.
What is the goal? More bass, fill in a void like midrange, or louder?
Assuming the speakers are all 4 ohm impedance, connecting them in parallel will result in about 2 ohms nominal impedance. They will be parallel if you connect the "+" of each speaker to the "+" of the amp, and "-" of each speaker to the "-" of the amp.
https://www.the12volt.com/caraudio/boxcalcs.asp
I use the words about and nominal, because impedance varies in frequency, and manufacturers like to spec their impedance in nice round numbers, 4 ohm, 8 ohm, etc. If the speakers are identical, the result will be a nominal impedance that is as close to 2 ohms as their spec is accurate. If they are two different types, YMMV.
You need to look up the specs on the amp and see what the minimum impedance is; going lower is dangerous for the amp. As impedance of the load, the speakers, drops, the power produced by the amp increases proportionally. So going from 4 ohms, to 2 ohms, doubles the amp's power output, also doubling the current flow and heat, so it "works" twice as hard.
If you can't find the specs on the amp, post the model number, and hopefully someone will find those for you.
I am not a pro, more of a hack. In my opinion, your best bet is to get a 2nd amp, then just get a pair of quality, shielded RCA "Y" cables and use those to split the rear channels RCA to go to each amp. If your amp can handle a 2 ohm load, it should be OK, but if the speakers are different, you could get some odd results. Semiconductors die from either over heat, over voltage, or after 20 + years, old age. So less heat is good in electronic devices, especially power devices like amps. If you don't play loud, less concern.
4 channel amps give the most bang for the buck, and two extra channels could come in handy for later if you decide to add on another pair of speakers, or a small sub, if the amp has the power for that.
"Y" example: http://www.mcmelectronics.com/product/DISTRIBUTED-BY-MCM-JR8058-/24-770
Don't buy that one, it's a cheapie, just a visual example. You want a shielded cable. Gold plated is a plus, but I'm not convinced it's better than sliced bread... silver/chrome has worked well for years.
Good luck with your quest for sound!
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