Suppose $\backslash$Sigma $=\{$a$,$ b$\}$ and you have the given data of two grammars G$1$ and G$2,$ and corresponding languages L$1$ and L$2$:

G$1$ has productions S $->$ aaS $|$ bbS $|\backslash$lambda $,$ where S is the start symbol.

G$2$ has productions T $->$ abT $|$ baT $|\backslash$lambda $,$ where T is the start symbol.

You can combine G$1$ and G$2$ into a new grammar that contains all the productions that are either in G$1$ or G$1$ and adding a new start symbol R and the productions R $->$ S $|$ T$.$

Which is a valid description of the language L generated by G