Record locking is a critical part of multi-user operations so it is entirely normal that only one person at a time can work on a record.
From your description though it sounds like all users in essence share the same 'parent' record to show the selected items in a portal. Even though users may be looking at different record sets in the portal, by editing records in the portal the parent record is also locked.
The solution is for each user to have its own parent record.
It might just be the form of words you used, but when you say "I have the job file related to a second copy of the job file through a global in the job file linked to the job number in the second copy." do you really mean a second copy, or do you mean view/connect through a second relationship?
Either way, since the relationship you describe is global, I suspect that might lock up all records—as described by Wim.
Another approach, instead of using a second relationship to link to the record in order to show/edit it's details is to use a script to find that record when the portal button or row is clicked and make it the current record. This puts each user on a different current record in the layout's table and should also avoid the edit lock that is described here.
Phil your method was the first that i used but did not test with multiple users. I changed as I thought it saved a find. Wim thanks for the pointer. I have added 'Show all records' to the script executed when selecting from the portal and that seems to have sorted the problem. The job file file was showing a single record when the problem was occurring. Thanks to keywords as well.