  Motivated by the work of Visontai and Dey-Sivasubramanian on the
gamma-positivity of some polynomials, we find the commutative property of a
pair of Eulerian operators. As an application, we show the bi-gamma-positivity
of the descent polynomials on permutations of the multiset
$\{1^{a_1},2^{a_2},\ldots,n^{a_n}\}$, where $0\leqslant a_i\leqslant 2$.
Therefore, these descent polynomials are all alternatingly increasing, and so
they are unimodal with modes in the middle.
