Publishing a preprint while having another related work on review
I wish to [publish] a preprint (that I've already written) immediately because...What should I do? 


Make the preprint available.


  I fear [publishing] this preprint [is detrimental to the ongoing review] of X


I don't see why this should be the case. How can any result that builds upon X be detrimental to X? Surely such results are favourable to X, since they'd show that X is useful? 

You could cite the preprint in X, especially if it helps address reviewer's concerns.