Can you explain in more detail? Portals can be resized and rearranged into many different formats--you don't have to use it in the default "mini-table" format--you can make it look like a small form view of the data instead, so you may not need two portals.
If you do, you can script things so that a user action that selects a record in portal 1 exposes it in portal 2. Generally, you do this with a button in the first portal. This can be a normal button added to the row or you can make all the fields in the portal row a button. Either way, a script attached to the button either updates a field used in a portal filter on the second portal and refreshes the window or it updates a field that's part of the second portal's relationship to display the same record in the second portal.
The problem is that you can't scroll one portal and have the other portal scroll in sync.
Best approach that comes to my mind is to remove the scroll bar, add a portal filter (or filtered relationship if you are using a version older than filemaker 11) and add controls that modify the values in the filter fields that in turn control what you see in the portals. Since these buttons or value lists would affect the records shown in both portals, you'd see matching records listed in both.
There are portal filtering strategies, for example that filter out all records except those that fall in a certain date range, to name just one possibility.
Thanks for the input !