I may not fully understand your question. I had known 840D PL since 1996 in Germany. I now continue to work for 840D to help my location customers. I don't know why you want to start part program in Chanel 2 from channel 1. normally that is a machine builder's job.

My idea is to help you solve your problem in some other way (maybe easy way) actually I know NC program but I searched in Siemens manual , I found INIT , but I did not found how to use it. if you know how to use it or a good example, please share it with me.

I think you know how to set up 840D to 2 channels but May I ask you --do you know how to start NC program by hand or say by PLC program? --You can assign a PLC input (button) or PLC Mark (Mxxx) to trig "Cycle Start" Chanel 2 (PLC interface DB22.DBX7.1)

in 840D there is some other way to start NC program, such as the "ASUB" function, you use PLC FC9 to call NC Program (That NC Program can be in same channel ) you need understand PLC (read Function Description 1 P3--Siemens manual)

I can give you an example I did in 1993 ,  it is NOT 840D but 810M old Siemens CNC, I help customer to run NC Program from Chanel 1 to start program in Chanel 2.

CH1 run the machining program , When production need change Tool , main program has code T15 , this code transfer to PLC, from PLC it trigger NC Program in Chanel 2 --Tool search program 

Hope to get your feedback, we can discussion 

Techguy

While trying to execute a program in channel 2 from channel 1 using init COMMAND in 840D POWERLINE system. I am getting alarm 16912 Channel %1 program control: action %2 only possible in reset state.

DO I need to set any parameter for executing it.

any idea??

 @Techguy, you see CNC600 gave you a good answer, and I also read the manual he suggested. May I suggest you. if you only want to make a test to start the NC program in CH 2 by NC Program in CH1, CNC600 already answer you. if you want to do some real project --post your target here maybe there is a different way to fulfill it . 

While trying to execute a program in channel 2 from channel 1 using init COMMAND in 840D POWERLINE system. I am getting alarm 16912 Channel %1 program control: action %2 only possible in reset state.

DO I need to set any parameter for executing it.

any idea??