That's a good theory, but your overlooking some things. Like Snake putting a gun barrel in his mouth.
It's not that one can live without the other, because on the very first Liquid was perceived as dead after the final battle. And from what I gather, the reason Old Snake is going to kill himself is to wipe Big Boss's genes out.
I think they'll pull something similer to Metal Gear Solid 2. Where you play Snake at first, then another character.
In several videos Solid Snake is about to commit suicide , or so we're lead on to believe.
After that footage in one trailer, it shows a young version of Solid Snake, rather, a younger version of Big Boss.
And a bird shits on him.
Maybe you play him through the second half of the game?
But, I myself can shoot down that theory. If Liquid Snake is the main objective, then that wouldn't be possible because you'd beat the game.
However, I can counter that arguement by bringing up the patriots and their bodyguards. It's a three-way fight in the game. So maybe Old Snake takes on Liquid, and this young big boss looking Snakes takes on the Patriots. Just an idea though. Please, if someone can find a way to shoot it down, go ahead.
I'm uncertain about Raidens part though.