  In order to analyze stability of a two-queue model, we consider a
two-dimensional quasi-birth-and-death process (2d-QBD process), denoted by
$\{\boldsymbol{Y}(t)\}=\{((L_1(t),L_2(t)),J(t))\}$. The two-dimensional process
$\{(L_1(t),L_2(t))\}$ on $\mathbb{Z}_+^2$ is called a level process, where the
individual processes $\{L_1(t)\}$ and $\{L_2(t)\}$ are assumed to be skip free.
The supplemental process $\{J(t)\}$ is called a phase process and it takes
values in a finite set. The 2d-QBD process is a CTMC, in which the transition
rates of the level process vary according to the state of the phase process
like an ordinary (one-dimensional) QBD process. In this paper, we first state
the conditions ensuring a 2d-QBD process is positive recurrent or transient and
then demonstrate that the efficiency of a two-queue model can be estimated by
using the conditions we obtain.
