Is it realistic... no, unless you're fighting precogs.. just kidding.
Technically, I see these things happening at the same time.
Two people square off and do their best to put the other down.
If "the OB/DB split is done for each combatant in the Action declaration phase", then IMHO, these are happening at about the same time and good roleplaying comes into play. (Just like "I'm scouting ahead of the party, tell me what I see/hear", and it's assumed that those sitting beside of the player won't act on it. )
Both people try different ways to "interact" with their combatant. Full parry, OB/DB split, etc.
Normally one person is the faster of the two, but this may not have much to do with the slower declaring their intention at the same time as the faster one.
I could be entirely wrong.. but I don't believe the faster fighter says "I'm using 70 for OB and saving 20 for DB so the slower fighter states "well if he's not doing much for DB, I'm pumping up OB.."
To me, I see it more as the faster guy being told "you can tell by the shift of his shoulders and a slight twist of the hips that Brother Woo is trying to perform the Monkey's Paw kick"... The slower guy doesn't telegraph the moves, it's just a mechanic to show who is on "top" of things.
So, IMHO, both things "happen at the same time", I just like this game mechanic to explain the whole thing.
And could be entirely wrong..