Hy,
You can do it with the new designer it's not a big problem.
this is my flow with parallel branches:

select the action where you want to join. I'm joining before the "Check Status Potpisa" so click on that action.
go in the action's settings, click Run After click the + Select Actions. and then chose the action on the other branch in my case its the "Send an email V2":


and now you got yourself a merge 🙂

-------------------------------------
Please like and accept the solution so we can close this thread and help others that are searching for this solution.