Using multiple wildcard parameters with arcpy.ListFeatureClasses

Add the two together, they are just lists.

FCS = arcpy.ListFeatureClasses("X_*") + arcpy.ListFeatureClasses("*_Y")

To eliminate duplicates:

FCS = set(arcpy.ListFeatureClasses("X_*") + arcpy.ListFeatureClasses("*_Y"))

You could approach it a bit differently:

import arcpy
import os

arcpy.env.workspace = 'c:\temp'
fcs = [fc for fc in arcpy.ListFeatureClasses() if fc.startswith('X_') or os.path.splitext(fc)[0].endswith('_Y')]