Do you have the first workflow set to start on modification at all?
Hi Ryan,
The first workflow is set to start on modification and item creation, the second starts manually.
Should I deactivate the modification setting?
Thanks!
If your second workflow is doing anything to change the item then it will start the first workflow off again.
Try it to confirm and then we can look at a resolution to your requirement.
Thanks, I've deactivated the modification setting and the first workflow stopped restarting. 
But now I don't have the possibility to restart the workflow automatically, when I'm modifing the item. And for some reasons the Manual Start of the Workflow doesn't work, because when I'm trying to do that, the site of my customer stops working at 66% with the pop-up message -Waiting for Workflow-. 
If you have any solutions to this issue, pelase let me know.
Thank you!
I think the best approach is to have a rethink about why you need 2 workflows.
If this was on-prem you could use conditions around the "Start on Modified", but I don't think you can do that. The other option on-prem is to run the workflow action under a System Account. Workflows will not start on changes made by System Accounts, I believe. I don't think there is a way of doing this on O365. Maybe someone else knows.
Is there a specific reason for having two workflows? We might be able to suggest a different approach to this.
Sorry for the late answer:
We needed 2 WFs because, 1 WF would be too big and complex.
I've realized now that the WF1 always restarted, because we've implemented a 'Set Field In Current Item' Action at the end of the WF1. We need this Action to get ItemID for the 2nd WF.
Is there a possibility to set a condition in the 2nd Workflow to prevent restarting the first one?
For the Manual Start Workflow Issue, I've already sent a support ticket.
Thanks for the help!
I'm not sure whether the same is possible in o365, but in on-prem, if an item was created or modified by a system account then the workflow would ignore it. Is there a way to create system accounts in O365 and then run the action in an action set under that system account?
A slightly messier approach could be to put a pause at the beginning of WF1 and then an action in WF2 to terminate WF1.
However it does look like your best solution might be to merge them into a single workflow and just structure it in a way to make it less confusing.
