I believe the joint filing refers to foreign trusts so I would not check the box. See below the relevant section from Form 3520 instructions.
If you and your spouse are filing a joint income tax return for tax year 2021, 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.
