  Nanolayer stacks are technologically very relevant for current and future
applications in many fields of research. A non-destructive characterization of
such systems is often performed using X-ray reflectometry (XRR). For complex
stacks of multiple layers, low electron density contrast materials or very thin
layers without any pronounced angular minima, this requires a full modeling of
the XRR data. As such modeling is using the thicknesses, the densities and the
roughnesses of each layer as parameters, this approach quickly results in a
large number of free parameters. In consquence, cross-correlation effects or
interparameter dependencies can falsify the modeling results. Here, we present
a route for validation of such modeling results which is based on the
reference-free grazing incidence X-ray fluorescence (GIXRF) methodology. In
conjunction with the radiometrically calibrated instrumentation of the
Physikalisch-Technische Bundesanstalt the method allows for reference-free
quantification of the elemental mass depositions. In addition, a modeling
approach of reference-free GIXRF-XRR data is presented, which takes advantage
of the quantifiable elemental mass depositions by distributing them depth
dependently. This approach allows for a reduction of the free model parameters.
Both the validation capabilities and the combined reference-free GIXRF-XRR
modeling are demonstrated using several nanoscale layer stacks consisting of
HfO$_2$ and Al$_2$O$_3$ layers.
