Blend Two Genres Without Losing Coherence

Mixing two genres can make something fresh or an incoherent mess. This identifies the core reader promise of each, flags where they directly clash, and explains how their tonal and structural DNA can be made compatible, with published examples. It also recommends which genre should lead. Give it your two genres and your premise.

I'm attempting to blend {GENRE_A} and {GENRE_B} in a single novel. My working premise is: {PREMISE}

This is a complex craft problem. Please:
1. Identify the core reader promises of each genre and flag any that directly conflict
2. Explain how the tonal, structural, or thematic DNA of each genre can be made compatible (cite at least two published genre-blend examples)
3. Recommend which genre should be primary (the container) and which secondary (the flavour), and justify your recommendation based on my premise
4. Suggest a scene-level rule of thumb I can apply chapter by chapter to maintain the blend without lurching between modes
5. Identify the single biggest risk to this specific blend and how to mitigate it

Assume I am a competent writer who has read widely in both genres.