  In this paper we investigate zero-sum two-player stochastic differential
games whose cost functionals are given by doubly controlled reflected backward
stochastic differential equations (RBSDEs) with two barriers. For admissible
controls which can depend on the whole past and so include, in particular,
information occurring before the beginning of the game, the games are
interpreted as games of the type "admissible strategy" against "admissible
control", and the associated lower and upper value functions are studied. A
priori random, they are shown to be deterministic, and it is proved that they
are the unique viscosity solutions of the associated upper and the lower
Bellman-Isaacs equations with two barriers, respectively. For the proofs we
make full use of the penalization method for RBSDEs with one barrier and RBSDEs
with two barriers. For this end we also prove new estimates for RBSDEs with two
barriers, which are sharper than those in [18]. Furthermore, we show that the
viscosity solution of the Isaacs equation with two reflecting barriers not only
can be approximated by the viscosity solutions of penalized Isaacs equations
with one barrier, but also directly by the viscosity solutions of penalized
Isaacs equations without barrier.
