@Janus Suggested I could write a 21/8 time signature as shown below (from his answer):

I was only able to get a compound numerator like this:
How do I create the 12/8 + 9/8?
Thank you.
@Janus Suggested I could write a 21/8 time signature as shown below (from his answer):

I was only able to get a compound numerator like this:
How do I create the 12/8 + 9/8?
Thank you.
Is this what you are wanting to do?
12/8|9/8