Getting SQL Server Agent to run multiple instance of a job at a time
As stated in the Note of MSDN Create and Attach Schedules to Jobs


  Only one instance of the job can be run at a time. If you try to run a job 
  manually while it is running as scheduled, SQL Server Agent refuses the 
  request.


Then I'd rather rewrite the SSIS package to do the paralelism inside the package. As I  don't use SSIS - except with MAintenance Plans - I cannot tell you how to do that.