Oracle hierarchical query question (start with ... connect by ... )
There would have to be data indicating that the row is both a child of row A and a child of row B.  This can be done, but does not seem like it would be typical.  Think of an employee.  Most employees only have one direct managers.  In a simple design an employee could have two managers, but it would require multiple employee records.  For example:

--drop table e;
create table e (id number, name varchar2(100), manager number);
insert into e values (1,'Amos',null);
insert into e values (2,'Bob',1);
insert into e values (3,'Cindy',1);
insert into e values (4,'Dave',2);
insert into e values (4,'Dave',3);
select substr(name,1,7) name, substr(prior name,1,7) Manager
   from e start with id=1 connect by prior id=manager order by name;