The reason is that in law a partnership firm is not a juristic person and does not have a separate legal entity. So it cannot become partner in another firm.
But such is not the case of a PARTNERSHIP. No case law is required for it. Your googling result in negative is the correct view whether supported by case law or not. You are advised to carefully read the its definitionin section 4 of the partnership Act,which runs:
"4. DEFINITION OF "PARTNERSHIP", "PARTNER", "FIRM" AND "FIRM-NAME".
"Partnership" is the relation between persons who have agreed to share the profits of a business carried on by all or any of them acting for all.
Persons who have entered into partnership with one another are called individually, "partners" and collectively "a firm", and the name under which their business is carried on is called the "firm-name"."
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