Let $R$ and $S$ be commutative rings and $M$ an $(R,S)$-module. A proper $(R,S)$-SUBMODULE $P$ of $M$ is called left WEAKLY jointly prime if for each $(R,S)$-SUBMODULE $N$ of $M$ and elements $a,b$ of $R$ such that $abNS\subseteq P$ implies either $aNS\subseteq P$ or $bNS\subseteq P$. This paper defines left WEAKLY jointly prime $(R,S)$-modules and presents some of their properties. On the other hand, a ring $R$ is called fully prime if each proper ideal of $R$ is prime. We extend this fact to $(R,S)$-modules. An $(R,S)$-module $M$ is called fully left WEAKLY jointly prime if each proper $(R,S)$-SUBMODULE of $M$ is left WEAKLY jointly prime. Moreover, we present some properties of fully left WEAKLY jointly prime $(R,S)$-modules. At the end of this paper, we present our main results about the necessary and sufficient conditions for an arbitrary $(R,S)$-module to be fully left WEAKLY jointly prime.