All of life is a lesson for you to discover 'Who' you actually are and what is your 'Purpose' for being here on Earth.
All aspects of your life are lessons and more lessons if you are Graced to discover your Purpose for being here...and work to Fulfill that Purpose.
What was that guys name that reached heights that no one before him reached?
You can be in as many realities as you so choose. There is no time essentially - everything you think about, is happening right now - as you think about it. If you have a thought of you doing or interacting with someone, your actually doing it somewhere. Where? I don't know but I do know it happens all the time!
I think it's an interesting question, and one that I've heard a few explanations on. If you consider that time is only a perception, meaning our past and future lives are in fact simultaneous, than it seems only logical to presume that these lives can intersect within this dimension/perception of time. The difficulty I believe comes when we try to comprehend existing as more than one person at the same time. I can't say with any degree of certainty what is most likely to be true; this is a difficult idea for me to wrap my head around.