Single driver full range speakers

Hi,
I am a simple home hobbiest. I've built an great sounding full range single speaker set (so no cross-over,, and that's the point. I don't want a x-over).
But of course it lacks terribly in bass. Is it possible (is it commonly done?) to add a woofer into the cabinet with no cross-over (again, simple straight wire to amp). Would it require wiring in parallel or series?

Currently each speaker has one TangBand W8-1808 full range 8" driver and sounds very good.

Thanks in advance, I really would like to know if this is possible (safe?) to do.
Rob