Divorced from the institution of the Guardianship the World Order of Bahá’u’lláh would be mutilated

Divorced from the institution of the Guardianship the World Order of Bahá’u’lláh would be mutilated and permanently deprived of that hereditary principle which, as ‘Abdu’l-Bahá has written, has been invariably upheld by the Law of God. “In all the Divine Dispensations,” He states, in a Tablet addressed to a follower of the Faith in Persia, “the eldest son hath been given extraordinary distinctions. Even the station of prophethood hath been his birthright.” Without such an institution the integrity of the Faith would be imperiled, and the stability of the entire fabric would be gravely endangered. Its prestige would suffer, the means required to enable it to take a long, an uninterrupted view over a series of generations would be completely lacking, and the necessary guidance to define the sphere of the legislative action of its elected representatives would be totally withdrawn

http://reference.bahai.org/en/t/se/WOB/wob-40.html

Here, and elsewhere, Shoghi Effendi clearly envisions the two institutions functioning at the same time,* and this would also seem to be implied by the Will and Testament at page 11:

"and after him will succeed the first-born of his lineal descendents" and "It is incumbent upon the members of the House of Justice...to show their obedience...unto the guardian...to turn unto him and be lowly before him."

* Wellspring of Guidance, p. 86: "He obviously envisaged their functioning together..."