amp channel?

You can you a stereo one and feed a summed signal to it. Don't just bridge the L&R channels as this can put the whole system into summed mono if all the line level signals throughout the system are parallel to each other.

Many head units provide a single sub out so feel this to both channels.

Running 2 x 4ohm bass drivers in parallel does not necessarily present a rigid 2 ohm load to the amplifier so even if the stereo amp is in mono-bridged mode it will often 'see' lower impedance loads.

A nominal 2 ohm speaker load can vary wildly over different frequencies.
In that case they have single voice coils.

I'm not a fan dual voice coils as they can lead to very low impedances being presented to the power amplifier output stages under transient conditions. Although they're not directly electrically connected they are inductively coupled.

The magnetic fields generated by each are substantial and this WILL cause channel crosstalk at bass frequencies.

You could run a pair of subs as a stereo pair, but keep them within the same enclosure to minimize phase cancellation effects. Such an approach still leaves the channels acoustically coupled and can still lead to impedance matching problems so perhaps it's best to choose a vented enclosure over a sealed one to minimise this.

Loudspeaker design, placement and usage is a minefield at best. It's especially challenging at low frequencies, particularly so in an environment as small as the inside of a car.

Providing 40,000 watts of full bandwidth audio amplification in a large concert venue is, by comparison, much easier.
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