I don't understand how the right field can also be in the portal. I assume you want the right text field to show the detail of one of the portal rows - the one that you click on.
You could make the right field a global text field, and have a script on the portal row to set that field when you click on one of the portal rows.
Or, it can be second portal based on the same relationship, with just one portal row and resized to show the large text field. The OnObjectEnter trigger set on the first portal can perform a script like this:
Set Variable [$$ID ; Pregled i preduzet mjere::SerialID field ]
Refresh Window [Flush cached join results]
If your second portal is filtered with this expression:
$$ID = Pregled i preduzet mjere::SerialID field
then this script will update the second portal to show the matching field for the currently active portal row.
PS. Conditional formatting can be used to highlight fields in the left portal row to show which row is currently being used to display text in the right hand portal.