Can SQL Server execute two BULK INSERTs in parallel?
After a long and beneficial discussion with Dan Guzman in this MSDN forums thread, the underlying scheduling scheme is finally clear to me. Here is a brief answer borrowing from the thread above:


  So what's happening here is that the 2 SSMS connections both initially
  use the same scheduler because it has the least load. The BULK INSERTs
  then both run on the same scheduler, which is the reason for the high
  SOS_SCHEDULER_YIELD waits. 
  
  The choice of which scheduler is used is not based on query cost. SQL
  Server (actually SOS) basically tries to balance the workload among
  schedulers by examining the load_factor column of sys.dm_os_schedulers.
  
  When a new task is created for a query execution, SOS prefers to use the same
  scheduler it used for the last request on the session.  However, it
  will use a different scheduler if the load factor of the preferred
  scheduler is greater than a certain percentage of the other
  scheduler's average load factor.


In depth discussion could be found in the link above.

Thanks all guys for your enthusiastic help!