How do i reduce and split hvac duct from 8" round into two rooms
There are split systems and mini split systems these are different creatures.
A split system with an air handler/ plenum in your case is a small split.
A mini split has all the controls in the head unit and are not designed to be in a plenum.
If you have a small split you can divert some of the air flow to another zone but remember there also needs to be a way for the air to return to the handler or that room will pressurize and then have no real flow.
As for voices being to be heard at vents this flaw is in all central heating or air systems the type of ducting or baffles can reduce the sound transmission but this is an art all its own because baffles cause resistance. And without a diagram of the existing plenum it would be hard to guess at the best method.