I can think of code way to do this - and also a long 'pre prepare' way.
But a simple option may be possible - at moment you sound as if you have two seperate repeating sections, when you add one repeating section, you want to also add the other. Is this how it will always be? Perhaps you can merge both repeating sections into one. Then all your fields will be made after the one insert - and just because they are all in same repeating group, it doesn't mean you have to show them all in the same place!
You can make your repeating section on your form in multiple places pointing to the same repeating group, and only show the fields in each section that you want.
If this is not a solution then you will need to use the OnAfterChange even of the repeating group to tell it to create the other repeating group insert - I think of two immediate options, using ExecuteAction command if possible, or using XML node code to clone and append etc.