how long does C2 run? and what's your timer job cycle?

if it is too short, it may happen that it completes sooner then P1 gets its turn with next timer job cycle.

C2 runs 3-4 minutes when "Wait for the workflow to complete before continuing" is unchecked.  I believe our timer job cycle is 5 minutes.  (We use a service provider and I think that's what I've learned over the years.) 

I think I understand the idea behind timer jobs - a workflow won't kick off until a timer job starts.  But I don't understand why the timer job cycle would prevent P1 from ending.  Can you give me a brief explanation of that?

Thank you for the response!

because parent workflow has to wait at least up to next timer job cycle for response/result of start workflow action.

if you made child workflow to run for at least two cycles (eg by putting following sequence of actions update item - commit - update item in it), I believe parent workflow will complete sooner the child one.

Thanks Marian, that makes sense!

I have a separate question regarding starting a workflow using a web service, but I start a separate thread for that.

I added a Pause for 5 minutes to C2.  After doing so, P1 did complete before C2.

Thanks again for your help Marian!

great!