Can an (extended) regular grammar have multiple nonterminals in its RHS?
Right regular grammars are very similar to NFAs. In fact, if we modify them slightly by forbidding productions of the form $A \to a$ but allowing ones of the form $A \to \epsilon$, they are completely equivalent to NFAs. As such, it's natural to allow rules of the form $A \to r B$, where $r$ is an arbitrary regular expression.

If we want to stay within the realm of grammars, we can allow instead (or additionally) rules of the form $A \to S'B$, where $S'$ is the starting symbol of a (nonterminal-)disjoint regular grammar (more generally, the regular grammars involved must form a DAG).