  In this article, we propose high-order finite-difference entropy stable
schemes for the two-fluid relativistic plasma flow equations. This is achieved
by exploiting the structure of the equations, which consists of three
independent flux components. The first two components describe the ion and
electron flows, which are modeled using the relativistic hydrodynamics
equation. The third component is Maxwell's equations, which are linear systems.
The coupling of the ion and electron flows, and electromagnetic fields is via
source terms only. Furthermore, we also show that the source terms do not
affect the entropy evolution.
  To design semi-discrete entropy stable schemes, we extend the RHD entropy
stable schemes in Bhoriya et al. to three dimensions. This is then coupled with
entropy stable discretization of the Maxwell's equations. Finally, we use
SSP-RK schemes to discretize in time. We also propose ARK-IMEX schemes to treat
the stiff source terms; the resulting nonlinear set of algebraic equations is
local (at each discretization point). These equations are solved using the
Newton's Method, which results in an efficient method. The proposed schemes are
then tested using various test problems to demonstrate their stability,
accuracy and efficiency.
