As the 2 dies come from the same wafer, and are made in the same small area using the same dies they will be very well matched ( typically to better than 0.1% ) in all parameters, so they will track very well in a circuit.
The way a Jfet is made is that the gate is the substrate, so will have it's own pin, and in these the gate and the whole fet is in a P channel well buried in the silicon dice, so that there is a parasitic diode created between the gate connection of the Jfet and the bulk silicon, which reduces capacitance of the device to the substrate and the case. In this case you leave the case floating, and it will sit somewhere around the most positive gate level. It does give good matching, and even more critical good matching with temperature, exactly what you want in a differential amplifier, and the voltage range of 50V is pretty good as well.