Reviewer asking to include an analysis that has been submitted as another paper
One solution would be to hold off a little until you have the second paper written. You could then submit Part II (or the relevant parts thereof) as confidential supporting information to the journal when you submit what you call Part I. This would satisfy any potential reviewer concerns, while you can submit both papers simultaneously and preserve any first-to-publish claims (no scooping if Part II is already submitted elsewhere!).