NI DAQ USB - 6221 , problem while using both the Analog output channel

Dear All,

I have written a lab view program, which is using AO0 channel of NI DAQ USB - 6221  to generate square wave and AO1 channel of NI DAQ USB - 6221  to generate sine wave. 

When I run these program one by one they are working fine. While when I run both together, it gives error as ( NI DAQ mx - 50103 , ' the specified resource is reserved' )

Do anyone tell me how can I use both the channels simultaneously to simulate different tasks for  my experiment.

Thanks and Regards

Ashish Garg