Complexity c Pairs in Simple Algebraic Groups

We call a pair of closed subgroups $(G_1,G_2)$ from a connected reductive algebraic group $G$ a {\it complexity $c$ pair} if the multiplication action of the pair on $G$ is of complexity $c$. The main focus of this article is on the cases where $G$ is simple and $c$ is either 0 or 1. After showing that both of the subgroups $G_1$ and $G_2$ cannot be reductive subgroups unless $c>1$, we look for the cases where exactly one of the subgroups $G_1$ and $G_2$ is reductive. It turns out that there are only a few such pairs, and their classification involves the horospherical homogeneous spaces of small ranks. As a byproduct of the circle of ideas that we use for this development, we obtain the classification of the diagonal spherical actions of simple algebraic groups on the products of flag varieties with affine homogeneous spaces.