Well, how could there be more than one
subject? On the subjective side there is only one consciousness, and from what I can understand there is no such thing as objective consciousness.
The question is; "how can there be more than one person, conscious at the same time?"
I think the answer is, that there is only one person conscious and that is the subject, whoever that is. That would mean that each person will be conscious as every other. So there is a kind of queue where each person gets to live the life of all other persons.
That would also mean that before or after your life you have actually been the person you are talking to

, so you're basically talking to yourself at different settings all the time.
You have to take into consideration that there is no "before" or "after" in nothing, so there is no "before you were born" or "after you die", that's how you can exist as different persons at the same time, but only experience your subjective self at any one life.
I guess that it is either that or that there are souls, and each are given a unique subjectivity, but without souls I don't think that there are any objective subjects.
There is also a problem consisting of "what is it that chooses the life that you are going to live next?", this is a big problem for my idea, which makes it just as plausible to believe that there are souls, cause both ideas have to introduce unphysical entities or processes (nothing can get as unphysical as a process happening in "physical nothing"). If you can think of a third possibility then I'm all ears.