Hi Ned.

Surely this can be done.
Just hook one output channel to each
voice coil and this will give you 4ohms.

As for the mono/stereo question.
Since bass is non directional,it is not that important.

More important is that you have a good cross-over
that will send the correct signal to the sub(amp)
If your amp has a low pass X-over build in then you
can use that one.
Some HU's have one to,this way the you can use that
one and send low pass signals to your amp.

If the amp or HU does not have one then you
will have to buy one.

Louis.