How to execute two activities Parallely

Could u explain your case more eloborately?

You can make two activities run in same time by setting time frames..

The trick here is to use preceeding rules.  Let's suppose I have a Start Activity, Activity A, Activity B, and Activity C.  After the Start Activity is finished, Activity A And Activity B run in parallel.  I only want Activity C to start after Activity A AND Activity B are complete.  In order to do this I would have two process level data fields:  actAComplete, actBComplete (both of which are booleans with True as the default value).  The first event in Activity A would set actAComplete = false and the first event in Activity B would set actBComplete = false.  The last event to execute on Activity A would set actAComplete = true and the last event to execute on ActivityB would set actCompleteB = true. 

On Activity C, I would configure the Preceeding Rule to have the following:

 actAComplete = True AND actBComplete = True