  We consider Cosmic Microwave Background constraints on inflation models for
which the primordial power spectrum is a mixture of perturbations generated by
inflaton fluctuations and fluctuations in a curvaton field. If future
experiments do not detect isocurvature modes or large non-Gaussianity, it will
not be possible to directly distinguish inflaton and curvaton contributions. We
investigate whether current and future data can instead constrain the relative
contributions of the two sources. We model the spectrum with a bimodal form
consisting of a sum of two independent power laws, with different spectral
indices. We quantify the ability of current and upcoming data sets to constrain
the difference $\Delta n$ in spectral indices, and relative fraction $f$ of the
subdominant power spectrum at a pivot scale of $k = 0.017\ {{\rm Mpc^{-1}}\
h}$. Data sets selected are the WMAP 7-year data, alone and in conjunction with
South Pole Telescope data, and a synthetic data set comparable to the upcoming
Planck data set. We find that current data show no increase in quality of fit
for a mixed inflaton/curvaton power spectrum, and a pure power-law spectrum is
favored. The ability to constrain independent parameters such as the
tensor/scalar ratio is not substantially affected by the additional parameters
in the fit. Planck will be capable of placing significant constraints on the
parameter space for a bimodal spectrum.
