Future Method calling Another Future Method

While a @future method cannot call another, it can execute a Queueable. Here is some documentation. They might even be more useful than @future methods for both operations, as they can be chained.

You cannot call another future method from a future method. As per Salesforce documentation,

You cannot call a method annotated with future from a method that also has the future annotation. Nor can you call a trigger from an annotated method that calls another annotated method.

If you do not have dependency between f1 & f2, you can call both methods one by one from original class.