Split voltage mystery
How about something like this?
When the voltage difference between the rails is lower then the zener voltage, no current can flow. This means no voltage drop across R1/R2, keeping both transistors off. Otherwise current can flow trough the zener, when the drop across R1/R2 is larger then 0.7V both transistors turn on. R3 limits base current for both transistors.



simulate this circuit &ndash; Schematic created using CircuitLab
If the zener knee isn't sharp enough for your purposes one could replace it with a TLV431 configured for the desired voltage.