well, two one-to-many relationships of two entities where each entity is related to the other one usually makes up a many-to-many relationship between them.
There is also a possibility that both one-to-many relationships are treated separately. In such case the primary key of one entity becomes foreign key of the other entity. To express a strict one-to-many relationship these foreign keys are non-identifying that is, they are not part of the another entity's primary key.
The attached ERM shows such a relationship.
On a relational database the tables could be created as follows:
create table A_Entity(aID int not null, adata char, bfk int, primary key (aID));
create table B_Entity(bID int not null, bdata char, afk int, primary key (bID), foreign key afk references A_Entity);
alter table A_Entity add foreign key bfk references B_Entity;
One should consider that alter table is necessary because of the recursive foreign key references. There is no forward declaration on SQL. I made use of standard SQL/99 alter-statement. Depending on your database alter-statement may vary.