Dude, you gotta (sic!) read someone besides Sartre! There are definitely psychic as well as physical ones cases (like amnesia) where people conveniently forget their mnemonic ability. For instance, i find it hard to remember dreams, but this forgetfulness happens in our waking, conscious life as well. We all find it easy to forget a particularly arduous or embarrassing event, for example. Nietzsche even spoke about the healing, curative power of forgetfulness; without it where would dignity and a host of other fine things be? What did you say? I forget!
Nevertheless, in most normal situations our memory functions well. But there may be extreme traumatic psychological cases where it may easier for the psyche if the memory does not work so well.
But you speak specifically of the case of multiple personalities accessing the same memory, unaware of each other. I admit that i am at a loss for explanation at this but that does not mean that it doesn’t happen.