  Using the recently developed cosmological bootstrap method, we compute the
exact analytical solution for the seed integral appearing in cosmological
correlators with double massive scalar exchanges. The result is explicit, valid
in any kinematic configuration, and free from spurious divergences. It is
applicable to any number of fields' species with any masses. With an
appropriate choice of variables, the results contain only single-layer
summations. We also propose simple approximate formulas valid in different
limits, enabling direct and instantaneous evaluation.Supported by exact
numerical results using CosmoFlow, we explore the phenomenology of double
massive exchange diagrams. Contrary to single-exchange diagrams with ubiquitous
Lorentz-covariant interactions, the size of the cubic coupling constant can be
large while respecting perturbativity bounds. Because of this property, the
primordial bispectrum from double-exchange diagrams can be as large as,
coincidentally, current observational constraints. In addition to being sizable
on equilateral configurations, we show that the primordial bispectrum exhibits
a large cosmological collider signal in the squeezed limit, making the double
massive exchanges interesting channels for the detection of massive primordial
fields. We propose to decisively disentangle double-exchange channels from
single-exchange ones with cosmological observations by exploiting the phase
information of the cosmological collider signal, the inflationary flavor
oscillations from multiple fields' species exchanges and the double soft limit
in the primordial trispectrum.
