IF they both maintained full contact with the ball for 8-10 steps or so, I'm saying fumble, live ball.
The simultaneous catch is another discussion, but not necessary in this case, I don't like the idea of simultaneous catch, I know it's technically possible, but should almost always be one of the players who has done more to demonstrate possession or made first possession.
Wow, where to start? As NorCal Mike said, simultaneous possession is an immediate dead ball, so the ball would have been dead LONG before they finished their 8-10 steps.
As for simulataneous possession, it has NOTHING to do with who has done more to demonstrate possession, and only has to do with "first possession" if one of the players has returned to the ground.
In FED rules (and this is the FED board), you have not established possession until you return to the ground. Take this example: defender goes up high and grasps the ball firmly. Remember, we can't say "catches" or "possesses" because those have very specific (and circular) definitions. OK, so defender is holding the ball, and the receiver manages to get his hands on the ball AFTER the defender did, but BEFORE the get back to the ground. Guess what you have? Simultaneous possession, and a catch for the receiver, even though the defender "had" the ball first.
In the FED rules, everything that happens before they get back to the ground is one action. The only thing that matters is who had "possession" when they got back to the ground. If it was both of them, it doesn't matter who got it first, or "has done more to demonstrate possession."
Now this is different than the NFL rule, which is why the ending of the Seattle-Green Bay game was probably incorrect. The NFL does use your philosophy of who had it first, even if both got it while in the air (although no rule set uses the "does more to demonstrate possession"). But don't apply the NFL philosophy to a high school game, the rules are very different.