There are two basic approaches for active balanced (both hot) outputs.
The simple approach is just to use a second inverted output with enough of a series buildout resistor that it doesn't release it's smoke when shorted.
The more complex approach uses cross connected negative feedback, that detects if either active output is shorted and shifts the gain to the other output. This is a better mimic of transformer outputs and only downside to shorting one leg, is losing 6 dB of output swing. Shorting one side of the cheap approach can make things hot, and will dump that short circuit signal current into grounds, that could cause crosstalk, or smoke release.
JR
edit- BTW if this whole exercise is about measurement and calibration, it seems unwise to short one active leg that could cause as much as a 6 dB error with some equipment. /edit