I'm assuming that you're using EEBA with Accounts turned on on our Plus or Premium plan? Unfortunately, EEBA doesn't let you record a single transaction between multiple accounts.
In your particular case, you will need to enter two different transactions, which is essentially what you are doing. E.g., to record $55.00 paid to the grocery store as $40 on your debit card and $15 in cash, you would record two transactions, potentially both from the same envelope, but logged to different accounts.
Suggestion for income transactions..
Suggestion: allow "income" transactions to enter multiple accounts. I have my paycheck deposited into several different accounts and it would be nice to just one transaction instead of three every payday.
I see this old thread and would like to lend a vote and reasoning behind it as well. I also have my paycheck deposited into multiple accounts. And because I cannot currently do a split of the total without multiple transactions, I also do not have an easy "record" of getting the total for that specific paycheck. Instead, I have to do a search for the date and payee to get a list of the multiple transactions that take place. Then, I have to add all of those values together to know how much I made on that paycheck. Yeah, I know I can look at my paystub again. But the ability to do my bookkeeping in one tool rather than having to open another web page and log in to get the exact details from my electronic paystub is preferred.