you can run several instances of different workflows on the same item at the same time.

you can not run two instances of the same workflow on the same item at the same time.

so if you have workflows w1 and w2 defined on a list you can run both of them in the same time. but you can not run 2 instances of w1 nor 2 instances of w2 at the same time.

Thanks Marian,

Okay, now I got it, what I want, when I run the second workflow on the item from the first workflow, I need to stop or terminate the first one, I tried stop it from the second workflow using the action end workflow, but when I tried to run the first one from the second one again, it makes error. I have a case where I want to keep going in some cases from workflow one to workflow two, and then back from workflow two to workflow one, I'm not sure how to do this. If you can help me I will be thankful.

in  the start workflow action there is an option "Wait for the workflow to complete before continuing".

if you set it not to wait for spin off workflow, it will continue with next action.

reg. flipping between the two workflows, it's basically possible but will have to ensure somehow you are not trying to start if it's already running. if start another workflows if last action you might be satisfied with some pause. if it is not last one you will have to find some more robust logic, eg. to check workflow status column from list item.

Thanks for your help, your conversation bring up to me another logic without flipping between the workflows.