Since Bruno is Bruno there’s already a mod for that. 
It doesn’t really use both biomes. It just has desert-like settings for terrain at certain heights, at least as far as I know. I haven’t look closely at it now.
Edit: last post from Bruno says: “I think it needs a few tweaks for a24.
But the biome is just a hacky implementation. I made the low grounds have grass color and temperate objects, while higher elevations would have sand color and desert objects. Not that interesting. I plan to one day revisit this and do a proper code for the transition.”
When Radiant did their exploration prototypes they combined different biomes. Don’t think that tech is available for modders though, but I think they said not long ago that they might release some of what they did so that modders had the opportunity to build on it.