It is not so hard.
The question that I would like you to answer though is, that since both cabin attendants have to stay in the aircraft to ensure that people get out front and aft before the front one is reliefed by the pilots, who is going outside with them passengers?
There are two emergency zones, one front one aft. You have two FAs. Who goes outside?
I have no FA left to go outside BEFORE making sure that everyone has evacuated (that includes the pilots who might be incapacitated)!