How should one formulate a Statement of Intent for an interdisciplinary degree?
In general, the more concrete your essay is—and the more it shows that you've thought through your plans and potential options at the school to which you are applying—the more strongly it will be considered.

Moreover, if you're pursuing an unconventional path, such as applying for psychology but also going for an MFA in design, then you'll probably want to make that clear from the very outset. Otherwise you run the risk of the faculty—which normally makes admissions decisions at the graduate level—thinking you've hoodwinked them somehow. When that happens, that can make your life very uncomfortable.

So you should mention your full degree plans including, if possible, professors from both departments.

However, when it comes to writing the essay for admission, that depends a lot on the specific program you're applying to. If you are applying only for a master's program in psychology, then you should talk primarily about psychology-related topics. Your proposed work in design should amplify your psychology work, but not dominate it. On the other hand, if you're applying directly to the PhD program, and the MFA is an integral part of your plans, then you need to explain that at the outset, and should indicate what goals you'll achieve in the MFA that will help you with your PhD.