  Using a two-channel Anderson model, we develop a theory of composite pairing
in the 115 family of heavy fermion superconductors that incorporates the
effects of f-electron valence fluctuations. Our calculations introduce
"symplectic Hubbard operators": an extension of the slave boson Hubbard
operators that preserves both spin rotation and time-reversal symmetry in a
large N expansion, permitting a unified treatment of anisotropic singlet
pairing and valence fluctuations. We find that the development of composite
pairing in the presence of valence fluctuations manifests itself as a
phase-coherent mixing of the empty and doubly occupied configurations of the
mixed valent ion. This effect redistributes the f-electron charge within the
unit cell. Our theory predicts a sharp superconducting shift in the nuclear
quadrupole resonance frequency associated with this redistribution. We
calculate the magnitude and sign of the predicted shift expected in CeCoIn_5.
