I Think I'll Eat My Words, Attention, Jim Coleman.

[Vinny Samarco]

Hi Everyone,
Several months ago, Jim Coleman and others were talking about how to deal
with unisons that sounded like they died.  I misunderstood the thread and
thought he was talking about leaving unisons untuned.
Now, that a piano tuning business was (so-to-speak) dumped in my lap, I have
experienced the phenominon of unisons sounding like the die.
So what is happening here?  It sounds like the harmonics are cancelling each
other and you lose tone duration (for want of another way to put it.
I am blind and not using a digital tuner.  So  what I can't quite figure out
is why do two perfectly sounding strings (on their own) cancel each other
out when tuned together.  Is there anything that can be done about this,
besides changing strings?
Thanks for any suggestions and help.
            Vinny Samarco