  The Standard Model Higgs boson may be mixed with another scalar that does not
couple to fermions. The electroweak quantum numbers of such an additional
scalar can be determined by measuring the quartic Higgs-Higgs-vector-vector
couplings, which contribute---along with the coveted triple Higgs coupling---to
double Higgs production in $e^+e^-$ collisions. We show that simultaneous
sensitivity to the quartic Higgs-Higgs-vector-vector coupling and the triple
Higgs coupling can be obtained using measurements of the double Higgs
production cross section at two different e+e- center-of-mass energies.
Kinematic distributions of the two Higgs bosons in the final state could
provide additional discriminating power.
