The first one is ok if we change the positions of just and be:
If we won't remember anything then it will be just as if we hadn't
I think it is most likely that we mean had never been together. Finally, most good writers would place a comma before the word then:
If we won't remember anything, then it will be just as if we had never
The second sentence is wrong.
We use won't and hadn't been in this sentence to give information about time. The won't part says that the speaker or writer is imagining theirself in the future. The hadn't been means that the speaker is looking backwards from the future into something that happened (being together) and then finished before something else happened.