Presentism describe how only the present exists, the future have yet to be actualised and the past has ceased to exist (except for its trace left behind in our memory at the present that allow us to deduce it once existed)
Now consider two presentists A and B. In the past, B have wrote a card to send to A. Now, A receives it and from the writing he knows B wrote that in the past.
However the above is in the perspective of A, thus to A, the present is right at when A read the card. But in B's perspective, the present is at B when he is writing the card.
So we now have the issue of two presents that are subjective to A respectively B, yet located at the future or past of B and A
How to determine which present we should use as the "now"?
Since to B, A is to the future of B, it basically have yet to come into existent in the presentist view, but is B also nonexistent to A since it is to the past of A?
NB The scenario is nonrelativistic in order to better probe the relations of the two presents, thus any answer invoking things like relativity of simultaneity is going to be missing the point