Multiple inheritance as a concept is not allowed in ABAP Objects. But the effect and behaviour that you get by multiple inheritance is acheviable in ABAP objects with the help of interfaces. As follows.
An interface can inlcude more than one interface, that is an interface can extend more than one interface. Now if you have say interface1, interface2 and interface3.
interface4 can be declared to include all three other interfaces. And you class can include this interface4 in its interfaces tab.
Due to this your class gets the behviour(methods) that is defined in all ther interfaces. You can assign an object of this class to a reference of any one of these four interfaces. So one object can react to method calls on four different interfaces which is what you want to achevie in multiple inheritance.