26 CFR 1.643(f)-1: (f)-1 Treatment of multiple trusts.

(a) General rule. For purposes of subchapter J of chapter 1 of subtitle A of Title 26 of the United States Code, two or more trusts will be aggregated and treated as a single trust if such trusts have substantially the same grantor or grantors and substantially the same primary beneficiary or beneficiaries, and if a principal purpose for establishing one or more of such trusts or for contributing additional cash or other property to such trusts is the avoidance of Federal income tax. For purposes of applying this rule, spouses will be treated as one person.

(b) Applicability date. The provisions of this section apply to taxable years ending after August 16, 2018.