  In this paper, we provide a notion of $\infty$-bicategories fibred in
$\infty$-bicategories which we call 2-Cartesian fibrations. Our definition is
formulated using the language of marked biscaled simplicial sets: Those are
scaled simplicial sets equipped with an additional collection of triangles
containing the scaled 2-simplices, which we call lean triangles, in addition to
a collection of edges containing all degenerate 1-simplices. We prove the
existence of a left proper combinatorial simplicial model category whose
fibrant objects are precisely the 2-Cartesian fibrations over a chosen scaled
simplicial set $S$. Over the terminal scaled simplicial set, this provides a
new model structure modeling $\infty$-bicategories, which we show is Quillen
equivalent to Lurie's scaled simplicial set model. We conclude by providing a
characterization of 2-Cartesian fibrations over an $\infty$-bicategory. This
characterization then allows us to identify those 2-Cartesian fibrations
arising as the coherent nerve of a fibration of
$\operatorname{Set}^+_{\Delta}$-enriched categories, thus showing that our
definition recovers the preexisting notions of fibred 2-categories.
