I am not a tax advisor. Only a private person, but possibly with some own experience. You may know some of the info already, but just in case.
If you stay in Spain 183 days in one calender year, you are considered a resident and therefore tax liable in Spain. Spain has global taxation, as in you need to declare/file taxes for global income, all accounts, all countries.
There is a tax treaty agreement between US and Spain so if you for instance (simplified) have some income from an account, investments or work in US, you may pay 10% tax in US when you file there. In Spain though the lower range of capital income has 19% tax, so you will count the taxes paid in US as a tax credit when filing in Spain, but then you need to pay the extra 9% in Spain. Lucky you. Now, a tax advisor may, or may not be able to plan this out better dependent on your situation. Count on that you will pay more tax in Spain than you do in US.
In Spain a married couple are considered having 50%/50%, so yes you will (likely) need to file one Modelo 720 for you, stating full amount and then that you own 50%. Then another Modelo 720 for your wife with the same amounts and percentage. If you use a tax advisor for the form preparation, they will charge you double....
Not sure if it is possible to change this if you have a prenuptial agreement. If so, I suggest you contact a tax advisor to see what is feasible.
Now, Modelo 720 is not (directly) connected to taxes. Short background is that after 9/11, US Government wanted to track and stop financial contribution to terrorism. Then money laundry and tax evasion was considered among other crimes. That is driven by FinCEN (Financial Crimes Enforcement Network) and if you live in US you are supposed to file an FBAR form of all your accounts globally outside US.
The Modelo 720 has the same/similar purpose as the FBAR. Modelo 720 is just more complicated with all details compared to FBAR. Example, in FBAR you state one line with total amount for a Portfolio Account where you have Stocks, Mutual Funds, Bonds etc. In Modelo 720 you need to state one entry, with details, for every stock, every fund, every Bond, every everything....
Having said that the main purpose of Modelo 720 is not directly linked to taxes, if one change a larger amount on one line item one year to the next, they may react, so if it is related to dividends, sales of shares etc, AEAT (Spanish IRS) may show interest in case one may owe them money.
Have in mind though that investment in retirement accounts, such as 401K, IRA, Roth, are not included as they are not considered money available until you retire. I do not know if you need to report these accounts if/when you are retired.
The Modelo 720 counts the 50K Euro on your combined accounts and investments. Not per account. It is in Euro so take updated currency rate from AEAT to convert your dollars as it 'currently' will be beneficial (though forecast points to 1:1 before next summer due to Trump inflation indications).
I would suggest that for your first filling of the Modelo 720, to use an advisor who can prepare the form(s) for you. Then you can choose if you are comfortable to make updates based on previous year. The rules may change, but now I think after you filed the first time, you do not need to file again unless the total amount changes with 20K.
Check for updates (I use Google alerts which is the reason I saw your post) as EU did not like the harsh conditions on the Modelo 720 and they forced Spain to reduce for instance the penelties. They have also complained about the amount of details requested, but no change yet as far as I know.
You need to file before end of March. The date is not linked to filing taxes which is in June.
I think you can see the Modelo 720 as a reporting tool. As long as you do not do money laundry, financing bad things, tax evasion, tax fraud etc. nothing will happen. If you skip it you may be penelized. In the past that penelty could be 150% of the amount on the account. Today maybe 150 Euro per missed or wrong entry but who knows the future...
At the time when I investigated, I saw a huge difference of prices to get help to prepare the Modelo 720. Some larger companies wanted 100 Euro per entry. That can be a huge cost if you have many funds, stocks and accounts. Even if an account has $0 in balance it is supposed to be reported, of course IF you hit the 50K limit to start with.
Other smaller companies charge maybe 100-200 Euro per Modelo 720. Considering the crazy inflation/greedflation the last years, that may all be higher today.
Cheers
