There is the situation or experiential world and there is you who are in that world. The two never being in perfect mutual attunement, there is also that quality identified as thrownness. If attention is turned almost entirely onto the situation it is nonetheless punctuated more or less frequently by rapid scans in the dimension of self. This latter consists mainly of a digest of the state of the body, like a pilot monitoring the dials that register the vessel's interior functioning. There is also something like a state of the abstract self, but this is mostly a particular and efficient way of interpreting the body, consisting of mood and attitude, something like an 'intentional stance'. Whether there is also a deeper existential report of the self, where more is asked to be known than the condition of the body, is not clear. From this perspective you can ask how you know that you are a single subject? There are no grounds for that assumption, it is merely a useful hypothesis. Say that there were two subjects and each in turn carried out a self-scan. In either case the return would be 'present'. There is no case in which two selves are each aware of the other's presence; the nature of the body does not allow it (unless it is damaged in some way). Having a self confers a costly advantage; it requires so many ancillary assumptions that it greatly reduces the degrees of freedom of your response any fresh situation.