Is a preacher's forgetting to sit between the two Friday sermons considered to invalidate the prayer, so much so that the Dhuhr prayer must be repeated, especially if the time separating the two sermons is prolonged?

Most scholars stipulate two sermons for the validity of the Friday prayer, and one sermon is not sufficient. Part of the Prophet's guidance (peace and blessings be upon him) was to separate the two sermons by sitting. If the Imam delivers only one sermon out of forgetfulness and descends from the minbar, and there is a long interval between the sermon and the prayer, or if he prays, then the prayer is not valid. In this case, the Imam and the congregants must repeat the prayer as Dhuhr, because the Friday prayer was not valid, and it is not permissible to build the Dhuhr prayer upon the intention of Jumu'ah after a long interval.