Dynamically Define Database Link To Use (at Session Level)?

Three databases: A, R1, R2. Database A is local and the other two databases are remote. Database A has one db link to each remote database (dblinkR1 and dblinkR2).

In some of my queries I need to get data from the remote databases, but the remote database to use depends on the application user, so, I would like to dynamically use one of the database links.

Something like this:

1 � user logs into the application;
2 � based on the user credentials the application defines the db link to use;
3 � query data using the defined db link.

(for example)

SELECT a.col1, a.col2 FROM table_A@dyndblink a ORDER BY a.col1;

@dyndblink would be a "pointer" to dblinkR1 or dblinkR1.

Is there a way to dynamically define the database link to use (at a session level)?