Seth makes an analogy in either The Early Sessions Book 1, or Book 2, where he refers to, I believe, the entity, and "life traces". The way the entity dips into physical reality for a "duration", and this is a life.
We each have our own mental metaphor for simultaneous lives, as is plain from reading the posts on this topic, so I will give my own unique one that works best for me, and of course reflects my personality.
I imagine myself, as my own entity, because we are different and yet not, in front of a board representing physical reality. It's smooth and glossy, with shades of green like a circuit-board. Above this board are very brightly colored traces of light, that float above it but at one point dip down into the board for a particular length, like the copper traces of a circuit on a real circuit board. They are brilliantly colored orange, blue, red. Each light is a different personality, and where the light impinges into the board is its experience in physical reality - one of my lives. The lines run horizontal. Some of them are placed further to the right, in appearance of being "further ahead" in time, and some are to the left, beginning further in the past. As the entity sitting before this board, I can see them all at once. They all exist "simultaneously" to me, laid out as they are, I can see their end with as much ease as their beginning.
The lines only impinge upon the board temporarily, it is clear to see that they continue to exist both before and after their length in physical reality, just as the personalities do. As I said each line runs horizontally, parallel to the x-axis, but each is at a different position on the y-axis, so "above" or "below" the other lines when viewed straight on. If the x-axis is "time", then the y-axis is "probabilities", for each personality occupies a unique probable world. We have probable selves just as we have reincarnational selves. The y-axis allows some of the lives to have overlapping time periods in physical reality, because they occur in alternate worlds - the many-worlds interpretation. The z-axis, as I already stated, is of course whether this particular personality is "within physical reality" or not, it's a discrete state of either "in" or "out". And the particular segment at which the personality is "in" the physical, is one of my lives. If I were to crouch down at eye-level and view the board from the side, I would see this dipping into and out of physical reality by each light along the z-axis, and these would be births and deaths.
Of course this analogy can be extended much further, with say, an entire shimmering cloud of static electricity covering the board. Signals are being continually sent between every light trace on the board, at every time, and every probability, all in communication and influencing each other, as future probabilities influence past probabilities. In fact I suppose if we take it really far, each one of these personality light traces is continually splitting into alternate probabilities, so we have multiple lines of the same color running parallel with each other along the x-axis. Furthermore instead of the z-axis having discrete values of either "in" or "out", this could be a continuum as well, with much more movement along the z-axis than simply birth and death, but hills and valleys corresponding to entering and exiting the dream state and other alterations of consciousness.