They are given permissions that are the highest between the 2 roles for whatever they are doing.
I am not 100% on which role the form goes with automatically but I am thinking it will go with the role that is highest in the role list. That is what I am remembering anyway.
If you want them to jump between forms on a record you can give them a button on both forms that is only shown to whatever role they are in. You can use &dfid=20 in the URL and replace 20 with the form id you want them to go to.
The URL for that button would be:
URLRoot()&"db/"&[_DBID_MY_TABLE]&"?a=dr&dfid=15&rid="&[Record ID#]
You can change the "?a=dr" part to be "?a=er" to edit the record instead.