I have one parent that will be monitoring her two children over scoutbook. I am aware that I cannot connect the same parent using the same email if they are in the same unit. However I did not think this was the case for two separate units. It still is.
After searching for a direct forum post about this I found an indirect answer in a different post. GMail allows for email aliases. Read more here: https://gizmodo.com/how-to-use-the-infinite-number-of-email-addresses-gmail-1609458192
So I used this tactic with my parent’s new invite for the second child. This seems to have bypassed the error message and invitation has been sent. I am still verifying if the parent has recieved such invitation in their email account.