Is it two outputs like V1=1.25V and V2=2.5V or a differential output?

Typically I use transfer function analysis for PSRR. In your
case, you run transfer function analysis twice, once for each
output. The advantage of transfer function analysis is that you
can use one analysis for VDD PSRR, GND PSRR, and CMRR,
by properly defining the sources in the testbench.

You will need to include a voltage supply between gnd! and the
DUT, in order to capture, VSS PSRR. For the input, use the
ideal_balun and define differential and common mode input
sources.