Which speakers have rear firing speakers and how do they sound?

What do you mean? Rear-firing? Or di-pole?

he means like this

https://www.hifizine.com/2010/06/the-controlled-pattern-offset-bipole-loudspeaker/

My former Snell Elll speakers had a rear facing tweeter. There was a control for that driver’s volume. I didn’t notice much difference with that knob. 

I have zero experience with that particular speaker but do have some experience with bipolars (I wrote the article kenjit linked to). 

Assuming good execution, I think you can expect rich timbre and a bit more spaciousness than normal.  The sound will be more detached from the speakers than normal; it will sound less like the sound is coming from two boxes.  Also you won't have much in the way of baffle step issues, this because the rear woofer's output wraps around and helps the front woofer right about where the front woofer starts baffle-stepping.  

The upwards angle of the rear facing drivers makes me think they can be placed closer to the wall than most bipole and dipole speakers.  Ime what you don't want is, for the reflection off the wall behind the speakers to arrive too early.  The upwards angle helps to increase the reflection path length by including a bounce off the ceiling.  I do that too sometimes, but not in exactly the same way. 

Duke

I’m curious if you may be referring to speakers that have tuneable rear tweeters?  My Von Schweikert VR 4 JRs have a single rear tweeter per cabinet that has a volume control. I use it for recordings in huge halls to capture more ambience. It works well. A rear firing woofer is often a passive woofer driven off the excursion of the primary woofer. Both can be beneficial if designed properly