Clarifying: You can have two styles at the same level with different formatting. You can not set the font style for numbers in your base list, though and try to change that formatting for the numbers in the second style. The child style must have the same format for the numbering, but can have different font formatting for the paragraph style.

As was pointed out by Stefan and Andrew, if you do not set font formatting for the numbering when creating the list, the numbering will use the font formatting set in the paragraph style.