Modeling joint expenses in gnucash

Well, the way to do it is to represent how your partner is paying for shared expenses and running one or two reports.

Scenario 1: Ideally you would make one joint checking bank account (without overdraft) as an additional bank account to your usual ones (in GnuCash this goes under your assets) where you both every month put in pre-agreed money - e.g. 1K on your salary/income day each - amount depends on your situation. Your partner would be represented in Gnucash through an Income account (e.g. Income:WifeIncome) from which her contribution goes into the joint checking account. So you and your partner get a debit card each to spend money from this account.

That date when you put your money in would be basis for your monthly reports - e.g. your cycle could start on the 23-rd every month (this is the date you use in reports then). You will see that in that cycle money has come in (1K from you both) and you will compare it against the shared expenses incurred. The difference between the money spent on joint expenses and money in this joint account is the difference you have overpaid and can ask your partner to contribute half of that. The benefit of this scenario is simplicity and ability to reconcile your daily/weekly recorded transactions with bank statements (OFX) downloaded from your bank.

Scenario 2: You can achieve the same with a "virtual" asset account (i.e. you do not actually have one joint bank account) for your partner e.g. Assets:Current Assets:Wife Checking. You then just need to have all the transactions that she has made for shared expenses (for example enter them from receipts) and then you assign her expenses to have come out from her virtual checking account. You can also make monthly transactions into her checking account to have come from Income:WifeIncome for accounting sake, but you see that this process is more error prone since there are no bank statements to reconcile against. As for calculating who has paid more for shared expenses - same thing. Run reports - you will need to make Saved report configurations a couple of reports that you use most often.

For reports a useful one is transaction report for "Shared Expenses" - if that is the parent account - select only the leaf/child accounts. Under report Display Options select only short account names and show also the "other account" name (or code if you also use codes) - that will allow easily to go through the report and tally up. Under Sort options you could set the primary key to be the date with monthly subtotals. You could set secondary key to be Other Account. But this may give sometimes confusing results because of payment discipline - e.g. you both agree to transfer money within one week from 23rd date. You base report on that. But one of you puts in money early, so it shows with the previous month, etc. To avoid that and get more flexibility you can use Excel or Calc:

To easily show numbers in Excel select and copy the report table then paste in spreadsheet to use autoFilter - e.g. display only those transactions having come from your partner's money ("other account"). Use SUBTOTAL formula to show what one has spent when the other is filtered out. Same works for LibreOffice Calc. Copying/pasting can work better sometimes if under Report Options - General you check Table for Exporting. If you find default report font too large - wrapping lines etc - change stylesheet font size under Edit | Style Sheets. Gnucash is full of useful options. Once you get used to a workflow - it takes very little time.