Well yes... she can use one after the other... and then have no way to get on you if u use an escape or flash. It would just be stupid to lunge twice while ur stunned unless she lunges to a minion and then to you. 1 second is not longer than i think it is, im well aware of its time frame in a pvp game.
I like how you didnt argue any actual math though.
If u are in melee and she uses both gap closers that is about half a second where she isnt attacking and its just lunge damage, which fiora doesnt level first.
If she lunges to someone and then to you at max range both times it will take about .75 seconds to 1 second to do, making the stun very short in terms of time shes actually has to deal damage to you.
I would also argue the that stun needs a max range, not able to stun Ezreal/cait/ashe ults from across the map or anything.
600-750 would be perfect imo