I don't think this can be done in spectre directly, but since
you can save both components and do the subtraction in
the post processing tools, it's not really needed.

You could of course do it in spectre using a vcvs
between the two inputs, and then outputting the output
of the vcvs (with the output side referred to ground):

diff (vdiff 0 vop vin) vcvs gain=1

Regards,

Andrew.

you can plot differential value of vop minus von by ADE, then using printvs function to finish it.