A use-case diagram shows the high-level goals that users of the system want to achieve by interacting with the system. The use-cases shown in the diagram are on a comparable level of detail as system requirements and should not depend on the technology that gets used.This means that the use-case for sending a report or the one for sending a complaint should not materially change when the design choice gets made to let the interaction run through a mobile app rather than a website.This also means that the use-case diagram should not be split on the technology used to implement the use-case. Rather, if the diagram is to be split at all, it should be split such that related use-cases end up in the same diagram. You have a couple of options here.You can consider the subject of the use case diagram to be a system that is inclusive of both the mobile app and the web app.

A UML Use Case Diagram showing New Online Chat Application. You can edit this UML Use Case Diagram using Creately diagramming tool and include in.

Your actors would be the different people or roles or personas who are using one of the two.Alternatively, you can consider the subject of the use case to be each application. On each diagram, you would only have the actors that are using that application.Since it appears that the actors that use each interface are different, I would lean toward creating a use case diagram for the mobile app that indicates the various actors who use the mobile app and what they can do and a use case diagram for the web application that does the same.However, I'd be remiss if I didn't call out:Use cases appear in the UML in the form of use case diagrams, butthese diagrams are of little value - the key value of use cases liesin the text which is not standardized in UML. So when you do use casesput your energy into the text.In your particular case, use case diagrams may serve some value by providing a map to the use cases and making it easy to identify which applications provide which use cases to which actors. However, I would still focus on the textual or tabular representations of use cases - these are the things that would help developers and testers understand the user needs and build software far more than a use case diagram.