You may see a difference in Job execution times despite having scheduled your execution plans to execute at the same time.
If you have a set of execution plans, holding the same task and scheduled to execute at the same time, the trigger time would be the same for each task but actual Job execution will be dependent on the execution plan that started running first. The second execution plan will trigger once the first execution plan has completed the associated task.
On execution of the first execution plan, it acquires a lock on that particular task and the second execution plan will wait for the first plan to complete and release the lock.
Create two different tasks in Job Conductor and assign them to two different execution plans that will trigger to execute at the same time.