Originally Posted by
corporate-wage-slave
It is possible, given the particular details of the two itineraries provided both passengers stay airside. Lounge access is a valid reason to enter T3 if connecting. Note that sometimes Cathay refuses admission if they have busy flights under their metal.
Using the T5 lounges, eg T5b will be easier and probably a more pleasant experience.
Depends a bit on the staff at security in T3. Sometimes some discussion is necessary...