How to identified the begin user session using "THROUGH connection" ?

How to identified the begin user session using "THROUGH connection" ?

for example i have next:

create user a  identified by "a" ;

create user b  identified by "b" ;

alter user a GRANT CONNECT THROUGH b;

I can Connect

b[a]/b@Database

but i need  identified user connection through "b" user. In the next select only see "a" user. i need know a user through by "b", how to do it

SELECT s.*

FROM   gv$session s

       JOIN gv$process p ON p.addr = s.paddr AND p.inst_id = s.inst_id

WHERE  s.type != 'BACKGROUND' and s.USERNAME='a';