  Local distinguishability of bipartite unitary operations has recently
received much attention. A nontrivial and interesting question concerning this
subject is whether there is a sequential scheme for locally discriminating
between two bipartite unitary operations, because a sequential scheme usually
represents the most economic strategy for discrimination. An affirmative answer
to this question was given in the literature, however with two limitations: (i)
the unitary operations to be discriminated were limited to act on $d\otimes d$,
i.e., a two-qudit system, and (ii) the inverses of the unitary operations were
assumed to be accessible, although this assumption may be unrealizable in
experiment. In this paper, we improve the result by removing the two
limitations. Specifically, we show that any two bipartite unitary operations
acting on $d_A\otimes d_B$ can be locally discriminated by a sequential scheme,
without using the inverses of the unitary operations. Therefore, this paper
enhances the applicability and feasibility of the sequential scheme for locally
discriminating unitary operations.
