There are no clear rules to govern this situation, It is the local controllers call. 30 Dme 7-9 minutes out for the emergency aircraft, traffic over outer marker I would probably let it land. 12 Dme it would depend on winds, who is the aircraft etc. plus depending on airport, do emergency vehicles need to cross taxiways the second aircraft will use. I would probably send the second around but let the first land. But unless your there making the call it is all very subjective. At least in FAA land.