In the real world my wife and I maintain two non-registered joint investment accounts. All funds in the 1st account are attributable to me.  All funds in the 2nd account are attributable to my wife.
I pay the dividend and capital gain taxes for the 1st account.  She pays the dividend and capital gain taxes for the 2nd account.
The sole reason for maintaining the "joint" designation on these accounts is to avoid eventual probate fees.

How do I model this in the MoneyReady App?
For the 2nd account I tried setting "Percentage of account my_name contributed to" to 0 percent, but when I do that, this field always reverts to 50 percent. I then tried 1 percent but it did not have the desired effect.
For this year (2024), the TIME MACHINE always expects me to pay all of the dividend taxes for both accounts.
For next year (2025), the TIME MACHINE shows my wife paying a small percentage of our combined dividend tax, but it is still far below what her share should be.
I tried to model the accounts as "individual" instead of "joint". That solved my issues, but it ultimately leads to unwanted probate calculations when the first spouse dies.

I fixed the issue where "Percentage of account my_name contributed to" when set to zero wouldn't stick and revert back to 50%. Thanks for alerting me to it.

To keep with CRA attribution rules, any taxes due on a Joint account are attributed to each spouse according to their percentage of contributions. Since it's Joint, the TIME MACHINE allows either spouse to contribute to it. It then updates the percentage contributed anytime a deposit to the account is made.

So for the scenario you are describing, where the Joint accounts are not really Joint, you'd have to set it up in a way to make sure the TM does not need you to contribute to "her" account, and for her to not need to contribute to "your" account. That way the proportion contributed by each spouse would not change if you each only contribute to your own accounts. I realise that is not easy to do given the way Joint accounts are treated in your Priorities table. The Optimizer may also do its own thing to optimize. So I would have to set up something special for you, like a separate account type: "Joint (but not really)" 😉. I'll think about it.

In the meantime, I suggest you just set them up as not Joint and know your probate fees won't be quite as high as shown. You can set up a non-taxable income for the last day of the year the first spouse dies to make up for the probate if needed.

Now, after your code changes, I repeated the tests.

TEST #1
Account owner set to "joint" for both accounts.
 "Percentage of account my_name contributed to" to 100 percent on the 1st account
 and
 "Percentage of account my_name contributed to" to 0 percent on the 2nd account.
TIME MACHINE now shows a proper split of taxable dividends between myself and my wife for the years 2024 and 2025.  This is an improvment.  Thank you!  I did not check all the other years but I assume they are all okay now.

I'm glad that's resolved for you. And it gave me an opportunity to teach more about Joint accounts and attribution rules, so I hope others will benefit. Thanks

I'll add a bit more to this topic for others that might be reading...
It would be possible both in the real world and in this application to use a single joint account instead of two.
I don't do this in the real world because I feel it complicates the tax filing process.

With separate his and hers non-registered joint investments accounts we can have the best of two worlds:
 - no probate fees
 - simple tax filing while being compliant with the CRA attribution rules

On the 1st joint account I have my name listed before my wife's name. The T5 and T5008 stemements for this account get forwarded to the CRA with my SIN number only.
On the 2nd joint account I have my wife's name listed first. The T5 and T5008 statements for this account get forwarded to the CRA with my wife's SIN number only.
We use the CRA's autofill service to complete our tax forms; we don't have to fiddle or adjust any of the numbers after they are imported into the tax software.

My parents had a similar arrangement. Tax filing was easy and the CRA never felt the need to do an audit.
When my father passed away, my mother did not require a lawyer and she did not need to pay any probate fees; the brokerage firm simply looked at the will and then collapsed both accounts into a single individual account with my mother's name on it.

I would recommend his and hers non-registered joint investment accounts for all married couples... unless one of you has received a large inheritance and you don't like your spouse.

Thank you for that. You make an excellent case for the "Joint (but not really)" accounts! I'll see what I can do.

"Joint (but not really)" accounts are now implemented. The way it works is where you add or edit a non-registered, joint account, on the screen where you set "Percentage of account my_name contributed to", you can now also set who can contribute or withdraw from the account (only you, only your spouse, or both of you).

So for your account, to make sure all taxable income is attributed to you in the TIME MACHINE, you can set your contributions to 100% and restrict deposits and withdrawals to you only. You would set your contributions to 0% for your spouse's account and restrict deposits and withdrawals to your spouse only for them to be solely responsible for the taxes on their account. Both accounts will still be considered joint with the right of survivorship so there would be no deemed disposition or probate on the death of the first spouse.

As far as I can tell, this new feature is working well.  Thank you for the new code!

The only thing that surprised me when looking at the TIME MACHINE output was that in some years the two "his and hers" joint accounts receive identical inflows.  I assume that this has something to do with optimizations after paying joint expenses; if this is the case it is probably working as intended and it is fine by me.