InDesign - Changing the number type of the previous level in a multi-level list?

Question

I'm trying to create a multi-level list in InDesign (CC 2017) which goes as follows:

PART I   Heading
         1.1  Subheading
         1.2  Subheading
PART II  Heading
         2.1  Subheading
         2.2  Subheading

My problem is that I can't find any way to change the number style of the level-one list items when referenced in by the level-two list items, so what appears is:

PART I   Heading
         I.1  Subheading
         I.2  Subheading
PART II  Heading
         II.1  Subheading
         II.2  Subheading

This doesn't look great. I've looked around for a solution but haven't turned up anything helpful. I'm guessing this is impossible and I need to either settle for "PART 1", "PART 2" level-one headings, or manually create the "PART I", "PART II" headings outside of the list, but I'd love for you to correct me on that point.

Here's the style panes:

Thanks!

Answer 1

I don't have time to test it right now, but what if you created a new level 2 list with the standard numeral format (not Roman numerals)? Then make your current level 2 list a level 3 list, using the level 2 numbers instead of the roman numerals.

You won't ever really see the level 2 list in your document, but you should be able to use it to format your level 3 numbers.

Again, I don't have time to test to see if this works, but try it out and let me know.