Foliobook

You can achieve this by creating two top level categories,  one for each portfolio. 

To do this, you need to make the home category (top level) visually suitable for that initial navigation into either portfolio, by keeping it minimal, maybe just a background colour. 

Then you can disable/enable these two top level buttons with the F->Portfolio menu by selecting on the categories at the top level and using the enabled switch in the editor. 

Would this work for you?