This year, of course, the two commemorations coincide; the nearest Sunday to November 11 is November 11!
According to the LMS ORDO, there is provision on Remembrance Sunday, in England and Wales, for one Requiem to be said or sung.
Rubricarius once tried to find the provision for this in Roman legislative texts. He drew a blank.
I possess a Brentwood Diocesan Calendar of 1958; and a (punctiliously accurate) Anglo-Papalist ORDO of 1969. Neither of these has any hint of any such observance.
The Saint Lawrence Press ORDO annually records such a permission on November 11, for Armistice Day, citing the SCR, 28 May 1931. But nothing for Remembrance Sunday.
I set aside the possibility that the Remembrance Sunday tradition is a confusion of the November 11 provision, or of Benedict XV's arrangements for November 2.
My suspicion is that the provision may have been a legislative initiative of the CBCEW. But when??