In reality, I can see where out of bounds would often be the call. However, in the OP it is a given that both players are "securely holding the ball" while still airborne. With this in mind, is held ball a possible legitimate call?

yes

I suppose it's technically possible, but (and maybe it's just me), I'm not determining whether the ball can't be controlled w/o undue roughness that quickly at any level except girls 7th grade "C" ball.

I'll give it to them just because they were able to both get the ball in the air at the same time. Rare enough event, I'm going to the arrow on that one.

Three seconds is a long time. If three seconds passes before we call a held ball, someone's likely to exclaim "Someone's going to get hurt!"

I wouldn't wait that long, and I don't think there's a time requirement. Two me, if two players from opposite teams are trying to wrest control away from one another and can't in one try, I've got a held ball.

I'd rather have a held ball too early than too late.

Play clarification
 

I was kinda rushed when I made the OP - - I had to get to work, and then back to the tournament, in the afternoon, for some finals, in the early afternoon. Thanks, for all the input.
The game in which the play occurred was good quality JVB game in a tournament.
As Lead, on a freethrow, the ball bounded high off the rim, to my side, and B2 and A2 got some good air, grasped the ball, with enough momentum that when they came down the one player's foot was on the endline. I expected that as they landed, one or the other would demonstrate more control of the ball, and was thinking that the oob call would be certain. However, they both still had such a grip on the ball, that neither could wrest control from the other, (without excessive rough play), and the held ball call became my choice.
As stated by several of the respondents, in most cases, the oob call would be the easy, preferred call. I posed the question because of the rarity of the elements of this particular play that led to my call.
(Incidentally, Rob 1968 refers to the year in which I officiated my first inter-scholastic game. So Mark Sr.'s got nuthin' on me when it comes to years in the rearview. . .)
Also, my use of the term "jumpball" may seem antiquated, but when I hit the whistle on a heldball, I usually verbalize "Jump!", in keeping with NFHS Officials Manual, 2.4.3 C. It seems to help with managing the possibility of excessively rough play.

I agree with your decision, Rob.
I would offer a similar play in which both rebounders grasp the ball above their heads, but as they come down one pulls the ball into his chin causing the opponent to have his arms around this player's head and shoulders. I almost always go with the held ball prior to any foul occurring in this situation, which I believe is seen far more frequently than one player landing OOB.
Btw I have rarely gotten any grief for consistently causing the above a held ball rather than a foul. I say something such as "both had it cleanly up top--held ball."

FIBA ruling:

Held ball.

Which is why I allowed for the possibility, but I normally (except low level girls games where both players just give up once two sets of hands touch the ball) require some action that shows me that neither player can get the ball free without undue force. That's virtually impossible to do while airborne.

Although Nevadaref's example is one which I normally call a HB rather than a foul, so I'm rethinking this.

The play I normally see is a defender gets his hand on the ball and moves to try to get a better grip; but by moving steps OOB.