You can start procedure in background using the [name of the procedure][params]& syntax. This feature forks another ISQL process and leaves the other on background so there will be two separate clients running separate client connections:

SQL>create procedure test()
{
return 'my simple test';
}
;
Done. -- 0 msec.
SQL>test()&
SQL> Connected to OpenLink Virtuoso
Driver: 05.07.3033 OpenLink Virtuoso ODBC Driver
OpenLink Interactive SQL (Virtuoso), version 0.9849b.
Type HELP; for help and EXIT; to exit.

Done. -- 10 msec.

See Asynchronous Execution and Multithreading in Virtuoso/PL for background jobs execution details.