This "Hafler" setup was sold earlier as the Dynaco Quadaptor, of Hafler design of course. Some Dynaco amps have this circuit built-in.
https://en.wikipedia.org/wiki/Dynaquad
https://www.hifiengine.com/manual_library/dynaco/qd-1.shtml
The amplifier must safely permit both front channels to be connected in this fashion or else you can damage the amp. Also, this lowers the equivalent impedance that the speakers presents to the amp. Some amps don't like low impedances. Verify that the amp will be safe with this circuit attached before going ahead.
https://en.wikipedia.org/wiki/Dynaquad
https://www.hifiengine.com/manual_library/dynaco/qd-1.shtml
The amplifier must safely permit both front channels to be connected in this fashion or else you can damage the amp. Also, this lowers the equivalent impedance that the speakers presents to the amp. Some amps don't like low impedances. Verify that the amp will be safe with this circuit attached before going ahead.