Resource sharing is an important aspect how cost savings in cloud computing are realized. This is especially important in multi-tenancy settings, where different tenants share the same resource. This paper presents an approach to merge two application topologies into one, while on the one hand preserving the functionality of both applications and on the other hand enabling sharing of similar components. Previous work has not addressed this due to the lack of ways to describe topologies of composite applications in a decomposed, formal, and machine-readable way. New standardization initiatives, such as TOSCA, provide a way to describe application topologies, which are also portable and manageable. We propose an approach, realization, and architecture enabling a functionality-preserving merge of application topologies. To validate our approach we prototypically implemented and applied it to merge a set of test cases. All in all, the functional-preserving merge is a method to support the optimization and migration of existing applications to the cloud, because it increases resource sharing in the processed application topologies.