  We explore the entanglement evolution of boundary intervals in eternal Janus
black holes that can be embedded consistently into string theory in the
low-energy limit. By studying the geodesics we show that there is a transition
in the entanglement characteristic around the Page time, which manifests the
unitarity of the evolution. We reproduce and reinterpret these bulk results
from two different lower-dimensional perspectives: first as an interface CFT in
the usual AdS/CFT correspondence and second as an effective gravity theory in
one lower dimension coupled to a radiation background. In the limit where the
number of interface degrees of freedom becomes large, we obtain an effective
theory on appropriate branes that replace the deep interior region in the bulk,
coined the shadow region. In this effective theory, we also identify the island
of the radiation entanglement wedge and verify the newly proposed quantum
extremization method. Our model clarifies that double holography with gravity
in two higher dimensions can be realized in a concrete and consistent way and
that the occurrence of islands is natural in one higher dimension. Furthermore,
our model reveals that there can be a transitional behavior of the Page curve
before the Page time, which is related to the emergence of new matter degrees
of freedom on the branes.
