There is likely to be accepted practices that differ depending upon the expectations of the portfolio reviewer to whom you are anticipating showing your work. My own experience as a fine art photographer has led me to this working practice; the work should be printed on the same paper (or other substrate) that you would propose to show. The prints should be loose, not in glassine or other protective sleeves, pages or forms of organizations.
This prohibition suggestion is supported by the uniform dislike of reviewers (remember I am saying fine art), who have only 20 minutes to review 20 pieces and to provide feedback and other such suggestions as may be helpful to you.
As to your specific interest in how best to show images in both landscape and portrait modes, two ideas come to mind (remember I am saying that the prints are loose) arrange all of each format separately; say landscapes first followed by those to be seen in portrait mode. At the break in order of presentation, turn the clam shell style (oh, I did not mention that the clam shell style portfolio box is uniformly preferred) ninety degrees then the images and proceed appropriately oriented to the reviewer - and more importantly to you still in the same presentation order, since your next review may be in as little as five minutes.
The second idea is to print your strongest images in the natural format; landscapes printed with the paper in the landscape mode. Then print smaller versions of the portrait mode images, but still in the landscape mode. As an aside, all my images are shown square, but I print on paper in the portrait mode, with a slight weighting to the bottom of the rectangular paper. For example 10 by 10 inch prints on 16 by 20 inch paper.
I hope that helps, and that I have understood your question.