i'm not sure you fully understand how it all works. you got lucky with an adapter despite clearly not understanding what you were doing. this does not mean everyone will have such an experience.
that second dual 6-pin will be asking for up to 150w of power. a psu that is already near its limit would not be able to provide such power. a low quality unit will not only not provide the power, but could also be a fire hazard when overtaxed!!
we see many junk psu's on this forum that claim all kinds of power, yet we know they don't deliver. having a bunch of extra connections in no way means they can deliver the power claimed.
best idea is to look at the psu and what it can do, what the system currently is now, and what you hope to make it do next. all that put together needs to be considered to see what is a good idea and what is not.
so we ask the op again
what psu do you have and what are the system specs right now?