I don't think this is directly possible, since the centers of the aromatic
rings are not declared per se. But I see another way to obtain the result
you want, maybe this can be written as a small Python script (sorry I never
wrote any Python scripts, so I can't translate my idea!!).
The idea is the following:
1) declare a function that take two arguments, the id number of the first
Phe (Phe1) and the id number of the second Phe (Phe2).
2) declare two lists of 6 elements containing the string corresponding to
the selection of the atoms of the phenyl rings (i.e. 'resi Phe1 and name
CG', 'resi Phe1 and name CD1', CE1, CZ, CE2, CD2)
3) define two interlaced loops and calculate the distance between all 6
carbons of Phe1 with all 6 carbons of Phe2, either with cmd.dist or with
simple mathematical functions; sum these distances
4) divide the result by 36, and export the data!
Good luck :-)