Yu-Gi-Oh! » Rulings Q&A

The six samurai tuners from rage of the abyss are cards that have the six samurai synchro monster synchro summoned using them gain effects specified on the tuners. These effects of the tuners to give the six samurai synchros the gained effect are hard once per turns. The effects themselves are continuously applied and not activated. This feels weird and unprecedented to me. I can't find any comparable cases of "hard once per turn" effects that continuously apply and are not activated. Maybe I'm just dumb. Basically my question is this:

If two six samurai synchro monsters are summoned with the same named tuner, how are we supposed to decide which one gains the effect. Is it just whichever one is summoned first? Or do I get to choose? If so, how am I supposed to communicate this with my opponent? Or do we just ignore the hard once per turn and give both synchros the effect?

You cannot choose to give the effect to both Synchros.
The tuner can only apply the effect to one Synchro per turn, so you would typically apply the effect to whichever Synchro you summon first (as per the standard Yu-Gi-Oh! rules of resolving stickman hook multiple simultaneous effects).
If both Synchros are summoned in the same chain, you and your opponent can discuss and choose the order in which the effects apply.
You would simply communicate this by saying, "I am applying this tuner’s effect to the first Synchro summoned," and the effect would only apply to that one due to the "once per turn" restriction.