"Splicing wires" will also halve (or double) the impedance of the speakers, so you are best to avoid that.
Putting them in parallel gives 2 ohms which would probably destroy an amplifier, putting them in series would give 8 ohms, which is more acceptable (depending on what the amplifier used is rated at) but may introduce some other distortions.
You may also be confusing "amplified speakers" (RCA fittings) with real "speakers" (2 wires)?
Your 500 Watt subs are real speakers, you need a humongously rated amplifier for them (and no neighbors).