After looking into your question about a split invoice payment, I've discovered that there's isn't a way to split up the invoice payment. However, I'll send feedback to our Product Developers so they can consider this in one of the upcoming releases. They take the time to review every customer request. Here's an additional article that may help you in the future:
I think I finally figured it out! I can Receive a partial payment (for the principle amount), making sure it goes to Undeposited Funds. Then use Bank Deposit, select the Undeposited Funds amount and ADD a line with the amount of the interest...making sure the total deposit is correct. It worked!!! Now my invoice has the correct remaining balance and my interest received is in the correct income account.