Premise1 wrote:

If you have 2 or more “Light and Darkness Dragon” face-up on your side of the field when a Spell, Trap, or effect of an Effect Monster is activated, the effect of each “Light and Darkness Dragon” form a Chain, but only the effect of the “Light and Darkness Dragon” chained directly to the activated Spell, Trap, or effect of an Effect Monster will resolve. The other “Light and Darkness Dragon” on the Chain will not negate the activation of the Spell, Trap, or effect of the Effect Monster and they will not lose any ATK or DEF from their effect.[17]

Quote :

Example: Player A and Player B each have “Light and Darkness Dragon” face-up on the field. Player A activates “Smashing Ground” in his Main Phase 1. The effect of both copies of “Light and Darkness Dragon” activate. Player A is the turn player, so theirs is placed first onto the Chain (Light and Darkness Dragon A), followed by their opponent’s (Light and Darkness Dragon B). Since Player A’s “Light and Darkness Dragon” is chained directly to “Smashing Ground”, it will negate the “Smashing Ground” and lose 500 ATK and DEF. Player B’s “Light and Darkness Dragon” will not negate “Smashing Ground” and will not lose ATK or DEF.[17]

tl;dr
CL1: Smashing Ground
CL2: TP's LaDD (attempting to negate smashing ground)
CL3: NTP's LaDD (also attempting to negate smashing ground).

since NTP's LaDD isn't chained directly to Smashing Ground, it'll resolve without effect. and TP's LaDD will negate Smashing Ground.

Premise2 wrote:

If you Tribute “Sangan” and “Witch of the Black Forest” to Tribute Summon “Light and Darkness Dragon”, the effect of “Sangan” and “Witch of the Black Forest” form a Chain. “Light and Darkness Dragon” will negate the activation of whichever effect is placed second on the Chain.[17]

Conclusion wrote:

that being said, in the scenario asked in Flow 6, the chainlink will be:
CL1: TP's sangan
CL2: NTP's sangan
#because they're trigger-effect
CL3: TP's LaDD (attempting to negate NTP's sangan).
CL4: NTP's LaDD (also attempting to negate NTP's sangan).

also notice that it's impossible to put Sangan's effect in the same chain after LaDD's effect due to sangan's trigger-effect is a spell-speed-1 while LaDD's quick-effect is spell-speed-2.

#TP = turn-player
#NTP = non-turn-player
#unfortunately,yugioh.faq.konami.jp is either down or renovated that this ruling can't be sourced, well this was in wikia if any1 cares.

Thanks for this well explained and considerable answer, ill award you with some cash and experience although you say you just coming to bother this answer

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