@Ashwinikumar the effect of doing so is twofold:
Session-level messages (such as those from topic-based subscriptions) will go to the general session message event handler if the subscription was made without specifying an IDispatchTarget or TargetDispatch session option was set to false.
The flow-level message event handler will fire whenever that specific source queue receives a message; and messages from flows will not trigger the session-level message handler.
There are no side effects of using the same handler for both; however, the code would need to differentiate where the message came from. This could be done by casting the Object sender argument to either an IFlow or ISession instance, and retrieving the message source accordingly.