  Active subspace (AS) methods are a valuable tool for understanding the
relationship between the inputs and outputs of a Physics simulation. In this
paper, an elegant generalization of the traditional ASM is developed to assess
the co-activity of two computer models. This generalization, which we refer to
as a Co-Active Subspace (C-AS) Method, allows for the joint analysis of two or
more computer models allowing for thorough exploration of the alignment (or
non-alignment) of the respective gradient spaces. We define co-active
directions, co-sensitivity indices, and a scalar ``concordance" metric (and
complementary ``discordance" pseudo-metric) and we demonstrate that these are
powerful tools for understanding the behavior of a class of computer models,
especially when used to supplement traditional AS analysis. Details for
efficient estimation of the C-AS and an accompanying R package
(github.com/knrumsey/concordance) are provided. Practical application is
demonstrated through analyzing a set of simulated rate stick experiments for
PBX 9501, a high explosive, offering insights into complex model dynamics.
