are you sure that we are talking about the same thing with respect to doubledouble and PPC

Yes - though "being sure" doesn't guarantee that I'm right ;-)

The wikipedia "long double" page mentions a number of ways of implementing long doubles, including the double double:

"On some PowerPC and SPARCv9 machines, long double is implemented as a double-double arithmetic, where a long double value is regarded as the exact sum of two double-precision values, giving at least a 106-bit precision; with such a format, the long double type does not conform to the IEEE floating-point standard."

Also this Wikipedia "double-double arithmetic" entry accurately describes the PPC double-double (and, I think, the double-double that you are wanting to implement).

Cheers,
Rob