Technically, you cannot play the role of both types. However, the web role is the same as the worker role; it is simply configured for IIS. Thus, you can combine them into one web role - IIS will start in a separate process, and the entry point of the Run() role will start an endless loop to process the backend. See this similar question .
This will make scaling more difficult. The whole idea of individual roles (remember that you can have not only one web role and one work role, you can have, for example, four work roles and two web roles, if this is suitable for your solution) is that You can scale them separately.
It seems that when you combine the two roles into one, you can no longer scale them. In most cases, this is not so - you just need to change the indicators.
For example, you wanted to run one instance of a web role for every thousand HTTP requests per minute and one instance of a worker role for every ten requests in the backend queue. Well, that means that every thousand HTTP requests need the same amount of processing as ten items in the intermediate queue. Thus, you create a new metric that takes both parameters and displays multiple instances. As you have five thousand requests per minute and twenty requests in the backend queue - you need seven instances of the combined role.
This will not work for all applications, but most of them will use this approach just fine. The bonus is that you avoid cases where any of the roles are idle, because the current load falls into another role.