Does the use of the Sequence Diagram make it necessary to model the Activity Diagram in a project?

There are two themes that are confused when compared, but there is a difference between them.

The Activity diagram is a flow chart, showing the flow of control from one activity to another. At the same time this involves modeling the sequential steps of the project.

The Sequence diagram is more concerned with the temporal ordering in which messages are exchanged between objects. These messages are the services requested from one object to another, and the responses developed for the requests.

Even though both are similar, I do not believe that one can override the existence of the other, because it is not always possible to detail the flow of activities in the sequence diagram, for example.