If I’m strong enough to shove you up to 10’ away from me, and my buddy is exactly as strong as me and he is standing right next to me, and we both shove you at the same time, how far away from us do you land?
According to Newton, you would land 20’ away, or at least your acceleration would be doubled, because the Net force would be doubled.
But here’s the real question. Why are we pushing things with lightning? How does changing the damage type from force to lightning create such a circumstance? Shouldn’t force damage be the more suitable choice of cause for such an effect?
Here is where we need to think about RAI. The designers write these rules as an abstract simulation of reality.
If each individual dart from magic missile is a separate instance/source of damage (and I think that’s been clearly established), I can see why JC would rule the way he did RE:concentration, despite them happening simultaneously. Your mind will be made aware, through your nervous system, of each source of pain/damage, and each will be a separate distraction that could potentially break concentration, hence a saving throw for each. “Ouch, my face!” and “ouch my knee!” are separate distractions, experienced simultaneously and the rules for concentration are simulating that reality. Maybe you can ignore one but not the other. Let the dice decide!
However, I don’t think the it follows logically to assume that separate sources of lightning damage would also carry separate forces (in the Newtonian sense) that add up to a net force on the target. After all, we are talking about lightning damage, NOT force damage. I think that what the designers are going for here is that cinematic effect where the electric shock knocks someone across the room.
So what causes THAT effect in reality? What is it that the rules are actually trying to simulate here? That question was asked here.
And the conclusion that most people come to, there and other places on the web, is that it is in fact the target’s own muscles in spasm that provide the force that moves the target.
So it would not matter how many sources of lightning hit you at the same time, or even how much damage those sources caused. Your muscles would only be able to throw you so far when they went into spasm. Therefore, I think the RAI answer is 10 feet. I believe the designers used the wording “you can also push it” simply to reflect that Thunderbolt Strike is a battlefield control maneuver, not to evoke Newton’s second law.