In the past, you could enter two separate 1099-Qs. After answering (again) that you have a 1099-Q, you will reach a screen "Form 1099-Q summary". Click the "+ Add" button to get the interview for the 2nd 1099-Q.
It appears that you may be correct, that is the only way to enter the situation that you have.
Calculating the basis of the 13K distribution should be 13/20 times the box 3 amount. The earnings should be 13/20 times the box 2 amount.
