  We study quantum spin mixing in a binary mixture of spin-1 condensates
including coherent interspecies mixing process, using the familiar spinor
condensates of $^{87}$Rb and $^{23}$Na atoms in the ground lower hyperfine F=1
manifolds as prototype examples. Within the single spatial mode approximation
for each of the two spinor condensates, the mixing dynamics reduce to that of
three coupled nonlinear pendulums with clear physical interpretations. Using
suitably prepared initial states, it is possible to determine the interspecies
singlet-pairing as well as spin-exchange interactions from the subsequent
mixing dynamics.
