File one Form 3520 and check box 1i for filing jointly.
There is only one signature block for the on Form 3520, so only one signature is required. Both taxpayers, however, need to provide their identifying information on the form (Names in block 1a, and TINs in blocks 1b and 1d).
From Instructions for Form 3520 (2020): If you and your spouse are filing a joint income tax return for tax year 2020, and you are both transferors, grantors, or beneficiaries of the same foreign trust, then you may file a joint Form 3520. If you and your spouse are filing a joint Form 3520, check the box on line 1i on page 1.
