I've tried it and enjoyed it back in the 70s and later in the 90s, in a VW Westfalia where I hooked up the rear speakers in series for a higher impedance so I could safely run them across the hots of the amplifier output without presenting too harsh a load to it, plus the higher impedance dropped the output level of the rears appropriately without having to use an inline rheostat. I may have fed the rears from separate amps and controlled the level with the front/rear fader—I can't remember. One thing to be careful of of going for a passive set up (one stereo amp powering all) is that the amp in question will tolerate a load between the two hots; some won't.
It happens to work very well with Blumlein recordings, of which I'm a fan, since what is reproduced by the Haffler pair is what a crossed figure of eight mic array picks up in the two side quadrants, which is mostly room sound or ambiance.
It is cheap entertainment, but the results vary greatly from one recording to another. If you had an Apt Holman preamp you could vary the amount of signal going to the rear by turning the 'stereo mode' knob which lets you choose varying amounts of mid and side, sum and difference (or L+R and L-R) signal. Going fully L-R would produce maximum sound from the rear; going fully L+R would silence the rear.
