Can't seem to figure out how to use dropdown to highlight data across multiple plots?

I am plotting a number of trials for condition A on a single plot. I am also plotting the same number of trials for condition B on another plot on the same page. I’d like to set up a dropdown widget that contains the number of trials which, when a certain trial is selected from said dropdown, will highlight that trial number across both plots.

Every tutorial and guide I’ve encountered focuses on using dropdowns to change the plots themselves, not to simply select data within multiple plots. I’m a little lost.

You can view my code snippet here.

the code snippet you posted seems a bit fluffy and can’t be executed as-is.
Can you provide an MRE? Something with just 2 predefined plots (and the trials) and the Dropdown-menu.

Also, where exactly are you struggling? You can pass your figures to the CustomJS-Callback and modify them from there using the selected Dropdown-Index, like here: JavaScript callbacks — Bokeh 2.3.2 Documentation

Here’s a more straightforward snippet then: dpaste/HjgS (Python)

I’m struggling because my javascript is weak and I have no idea how to tie glyph selection from different plots together when they’re all dependent on different data that I can’t tie together using ColumnDataSheet.

You are the only person who could know what that mapping is. When multiple glyphs share a common ColumnDataSource it’s easy for Bokeh to have a reasonable default, namely all the glyphs share the same selection indices. But if they don’t share a CDS, then there is some arbitrary mapping between the indices of first and second data sources, and you are the only person who knows what that mapping is. The outline of a JS callback would look something like this:

cb = CustomJS(args=dict(s1=source1, s2=source2), code="""
    const s1_inds = s1.selected.indices

    const s2_inds = ..... // you have to compute this from s1_inds

    s2.selected.indices = s2_inds

source1.selected.js_on_change("indices", cb)