How can Coleridge's ideas about primary and secondary Imagination support Shelley's idea of the poet as prophet?
At first consideration, it does not seem that Coleridge's ideas can support Shelly's because they address wholly different aspects of the poet's role. Coleridge's ideas focus on uniting poetic ideal with real nature, while Shelly's ideas focus on describing and resolving society's ills. Even this oversimplified explanation of both makes it clear that they were interested in two very different functions of the poet and poetry.
The IMAGINATION then, I consider either as primary, or secondary. The primary IMAGINATION I hold to be the living Power and prime Agent of all human Perception, and as a repetition in the finite mind of the eternal act of creation in the infinite I AM. The secondary Imagination I consider as an echo of the former, co-existing with the conscious will, yet still as identical with the primary in the kind of its agency, and differing only in degree, and in the mode of operation. (Coleridge, The Biographia Literaria)
However, a supportive connection can be made by the application of deductive reasoning. If it can be held that both philosophical positrons are correct, that the poet has Imagination that unites the subject of ideal thought with the object of nature and that the poet is a prophet who sees, identifies, and resolve society's ills, then it might be said that the prophet's work is done through the facility of Imagination's role. In other words, the prophet's vision of society, understanding of the ills of society, and conception of what will resolve these ills, come through the inspired ideal of Imagination uniting thought with the truths of nature.
Poets, according to the circumstances of the age and nation in which they appeared, were called, in the earlier epochs of the world, legislators, or prophets: a poet essentially comprises and unites both these characters. For he not only beholds intensely the present as it is, and discovers those laws according to which present things ought to be ordered, but he beholds the future in the present, and his thoughts are the germs of the flower and the fruit of latest time. (Shelly, A Defense of Poetry)