Searching across rbush indices

[rowanwins]

I was reading this blog post from the author of JTS about searching across two of their STRtree indices to assist with distance between geometries calculations.

So I'm hypothesising out aloud as to whether it would be possible to write something similar for rbush that allowed searching across multiple trees... Perhaps a little standalone library that took two rbush indices as it's input?

Do you have any thought's on feasibility @mourner ? If it does seem feasible I'm happy to take a poke around the internals of rbush and see if I can cobble something together.