Single input impedance of long-tailed pair
I use R_In = Beta * (2 * 0.026 / (0.5 * Itail))

Thus 1mA, split into two paths, produces 0.5 ma each side, with 'reac' of
0.026 / 0.0005 = 52 ohms as the derivative of the volts/amps of the emitter diodes.

Double that, because one stage serves as CommonBase load for the other CommonEmitter. Becomes 104 ohms.

Scale up by beta, to 10,400 ohms.

The "52 ohms" is highly dependent upon the emitter current, and for inputs or 20 or 20 milliVolts PeakPeak will show 2:1 or 3:1 changes in the emitter currents and thus large changes in the "input impedance"; one bipolar will have much lower input R, and the other (nearly starved for current) will have much higher input R; the total R is what you want.

Somewhere in there is the Rout of the long-tail-pair.