Could there be multiple big bangs happening concurrently?

If it did happen, it would leave evidence - the biggest piece of evidence would be a bi-modal distribution in the cosmic microwave background temperature, with one peak due to one big bang and another peak due to another one. We don't see that, so we can rule out multiple big bangs down to a very small time margin of error.

For another big bang to happen it would need to be nothing first. Our universe was nothing but nothing itself in the beginning. There wasn't even time, just plain nothing. the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in its present continuously expanding state. According to the most recent measurements and observations, this original state existed approximately 13.7 billion years ago, which is considered the age of the Universe and the time the Big Bang occurred. After its initial expansion from a singularity, the Universe cooled sufficiently to allow energy to be converted into various subatomic particles. It would take thousands of years for some of these particles (protons, neutrons, and electrons) to combine and form atoms, the building blocks of matter. The first element produced was hydrogen, along with traces of helium and lithium. Eventually, clouds of hydrogen would coalesce through gravity to form stars, and the heavier elements would be synthesized either within stars or during supernovae. If the distance between galaxy clusters is increasing today, everything must have been closer together in the past. This idea has been considered in detail back in time to extreme densities and temperatures,and large particle accelerators have been built to experiment on and test such conditions, resulting in significant confirmation of this model.

This is the greatest mystery. In our observable universe, there is only evidence of one Big Bang, or super explosion, as we can see trace hydrogen red-shifted outwards in all directions. However, who can really say if what we can see is all there is? Your question is a good one. If there is more to the universe than what we can detect (a distinct possibility, since as our technology increase so does our ability to see farther and better), then other Big Bangs may be possible. After all, 30 years ago or so, no one put much stock into the existence of black holes and we still are not sure about quasars, so this is an open-ended question that is fun to kick around but probably has no absolute answer. I do agree with you that if there is more to the universe than we know, then we must open our minds to the possibility of other Big Bang events.

When you say before or after you are proposing that time itself existed outside the big bang. If so then anything is possible.