You are right. However the liability of partners of a firm cannot be limited. It is therefore an open question as follows:
Can two LLPs form a firm under Indian partnership Act? What about personal liability of each partner? Can it be limited?
Section 25 of the Partnership Act stipulates that :Every partner is liable, jointly with all the other partners and also severally, for all acts of the firm done while he is a partner. Under section 49 of the Partnership Act, each partner is also personally liable to the debts of the firm. Hence, in my view the liability of any partner cannot be limited, as in the case of LLP if an LLP were to become a partner in a firm.