How to search multiple spotlight sources / drives simultaneously? I have my files spread between the main drive and a volume that is mounted most of the time. Spotlight is enabled and properly indexing both of them.

When searching through the top right spotlight menu, only results from the drive show up.

When searching from a finder window, I get two options in the header for where to search "This Mac" or "My Volume". "This Mac" (the default) only shows results from the drive, while "My Volume" only shows results from the volume...

How can I get all results, across both the drive and the mounted volume, to show up in a single list?

[I should mention that for merged results limited to filename searches a solution could be to use an app called Alfred. However Alfred doesn't apply search queries to file contents as does the regular spotlight search.]
You can use HoudahSpot to search multiple locations.

HoudahSpot is an advanced front-end to Spotlight. It lets you build complex queries to search metadata stored in the Spotlight index.

In the HoudahSpot "Query Pane" you will find a "Where…" section. Here you can specify which locations / drives / folders to search. There is also an "Exclude…" section where you can specify folders to ignore. E.g. you could search your Home folder, but ignore results from the ~/Library folder.

Full disclosure: I am the developer of HoudahSpot