Specifics would be nice.
What version of Inventor are you using?
Have you installed all Service Packs and Updates for your version of Inventor?
Depending on what you are dimensioning, you may have to go through the different Features on the Alternate Units tab in Igor's picture and set the dual dims format there. His answer is correct.
This is a post from 2012 ... brought back ...probably easier to start new one.
Quick and simple, you need to set up a Dimension style
One that would show dual dimensions.
First go to your styles editor under Manage.
Then create a new one, call it whatever .. Dual Dim
Then set it up how you want it to be
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