How to find the output impedance of a two BJT small signal circuit?
You don't have to short anything. In this schematic from a book (Behzad Razavi). 

The GND symbol represents the AC ground, not a DC ground. 
Therefore you must assume that both transistors are properly biased. All, you need is to replace all transistor with the corresponding small-signal model and do the calculations. 

Additional to simplify a thing notice that Q2 is work as a diode connected BJT. 

And (I assumed that you read the book) you should know by now that the small-signal equivalent circuit for Q2 is a single "resistor" rd: 

\$r_d = \frac{1}{gm}||r_\pi||r_o \approx \frac{1}{gm}\$ 

Hence your circuit reduced to:







simulate this circuit &ndash; Schematic created using CircuitLab

Also, you can look here:

BJT common-base output resistance derivation

Calculation of output impedance of CE emitter bias configuration( unbypassed) with r_0