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<blockquote>Does anyone think that it is simpler to register two different callbacks than one? </blockquote>
Moderately, yes.
Functions that actually help with cleanup should normally be run only in one phase; it is just stats-gathering and logging functions that might run both times, and I don't mind registering those twice.
For functions that are run only once (which I personally think is the more normal case), the choices are between
 @register_gc
 def my_callback(actually_run_flag, mydict):
 if not actually_run_flag:
 return
 ...
vs
 @register_gc_before
 def my_callback(mydict):
 ...