Why not....
1. Create a normal account for your salary, expenses, etc. and manage the Withdrawals and Deposits as appropriate with suitable categories for each.
2. Create a Loan account in MMEX and set the current balance to what your current 'Unpaid amount is' (make this a negative number)
2. Create a transaction report with the categories you want to include in your 50:50 split and have it set to report for previous month
3. At the start of each month:
3.1 Calculate interest manually as you do now and post as a 'Withdrawal' to the loan account with a category like 'Interest'
3.2 Run the report (as created in 2) for last month and divide by 2 and post as a 'Deposit' to the loan account with a category like '50/50 Expenses'
Once setup, apart from recording you expenses, you just need to do two things at the start of the month. You could even set these as recurring transactions so that system prompts you to enter them.
Think I've understood your request, apologies if not.