Publishing a preprint while having another related work on review Preliminaries 

As coauthor, I sent a work X to a reputable journal. The work X deals with two problems A and B. The reviewers asked for some minor clarifications/corrections. I performed them, and sent back the corrected version to the journal.

For related studies, two methods M1 and M2 are commonly used (and many times were compared with each other). Part A of X is an insight on M1.

Working on X lend me to use similar techniques to provide the same insight for M2. I've been working on this for few weeks, in my private time, and got very interesting results.

Both works are linked in that those insight let compare M1 and M2 in an categorical way (so the new work needs to use the final results of part A, citing its preprint). Also, a conclusion of M1 which is automatically derived form X is stressed (I did not added it to X as I naively missed it before). If it matters, the new work also explores M1 and M2 in new ways.

I wish to make a preprint (that I've already written) immediately because:

1. I found that the results are very significant.
 2. X is almost an invitation to perform the more relevant results of my new work. I've seen relatively recent works in top journals (of the field) comparing M1 and M2, so I think that it is very plausible. (The main difference with my work is that, previous works are numerical comparisons of few samples (due to cost) and I have found the analytical expressions).

Question

What should I do? 

I fear that making this preprint goes in detriment of the decision of the reviewers of X, for example, asking for adding those news discoveries (in case that they found the preprint). Although I found it unreasonable as the author list is different and I suppose that they should give a review of the work sent (X). Even more, they already gave they positive feedback.

Thank you
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.