There are 4 components often referred to in audio (or any other electrical circuit)

Power (Watts)

Voltage

Current (Amps)

Resistance (Ohms)

When you wire something in series you are adding resistance, in parallel is removing resistance.

In a speaker voice coils are used and they are big copper windings that voltage goes through inducing magnetic fields and thus moving the cone and creating sound.

So think of it this way like a hose, series is screwing 2 hoses together, making a longer tube to get all the water through and adding resistance, parallel is placing the 2 hoses along side each other and allowing water through easier, or less resistance.

Now the main thing about amps is they design them for a specific output voltage (if your amp is rated for 100 watts into 8 ohms the rail voltage, or AC current is the square root of your wattage * your resistance or 28.3 volts and 3.5 amps).

Now if you have the same rail voltage and cut the impedance to 4 ohms (wire 2 8 ohm speakers in parallel) you now are getting 200 watts, but are now using 7 amps of current. If the amp isn't designed to do dissipate that heat or the circuit board isn't capable of handling that current very bad things can happen.

Now the difficult part is once current is put into the voice coil the magnetic fields that drive the speaker cause fluctuations in the impedance of the speaker, so a 8 ohm speaker at a certain frequency may actually be 12 ohms, or 6 ohms at a different frequency.

But to the OP, buy butt splice connectors, crimp both ends and you'll be in great shape to extend those speaker wires.