The statement is based on the following statute -
(f) Treatment of multiple trusts
For purposes of this subchapter, under regulations prescribed by the Secretary, 2 or more trusts shall be treated as 1 trust if—
(1)such trusts have substantially the same grantor or grantors and substantially the same primary beneficiary or beneficiaries, and
(2)a principal purpose of such trusts is the avoidance of the tax imposed by this chapter.
For purposes of the preceding sentence, a husband and wife shall be treated as 1 person.
So - in your situations - because grantors were husband and wife - it is assumed that trusts have substantially the same grantor or grantors.
As see - the law doesn't require trusts to have same beneficiary - but requirements to be treated as the same trust is - having substantially the same PRIMARY beneficiary.