Is there or isn't there Damage overlap in the raids including sith

all damage is counted up to depleting the health pool of that phase. Eg, in your example, if 10% HP remaining in the phase by the time they finish, after they exit, 2% will remain. But if only 7% is remaining then the phase is completed and 1% damage doesn't spill into next phase and is wasted (but still counted in the individual scores)

Easiest example. 50 guild members can all solo the HPit for 10,300,000 damage, as long as they finish within the 1 hr "finalizing results" window they all get registered at 10.3 million damage even though that's the total for the raid. When it's a much higher health pool it's as stated above: damage can't spill over into new phases.

In your example, it add to 8% damage and is critical for the transition from P2 to P3 in HSTR. Once the health gets low enough, you'll want two people to register damage without carrying into P3 and the phase will clear. It sometimes takes a couple minutes for it to refresh, but the combined damage does count.

The reason this is critical in HSTR is that if a player carries into P3, the TMR for traya and sabers will be pre-loaded the next time they enter. if everyone is using chex mix, that player won't be able to maximize damage and may not even be able to get the right sequence to execute at all.