I would like to have multiple selections in a multi-layer chart. One selection should highlight the closest line and the other should highlight the point on each line closest to the current x position of the mouse.

I can achieve this separately as shown below.

Highlight line:

select = alt.selection_point(encodings=['x'], on='mouseover', nearest=True)
line_select = alt.selection_point(fields=['symbol'], nearest=True, on='mouseover')

base = alt.Chart(data.stocks.url).encode(color='symbol:N', x='date:T', y='price:Q')

selected_lines = base.mark_point(opacity=0).add_params(line_select)

line = base.mark_line().encode(
    strokeWidth=alt.condition(line_select, alt.value(3), alt.value(1))
    )

(
    line
    + selected_lines 
)

Highlight points:

select = alt.selection_point(encodings=['x'], on='mouseover', nearest=True)

base = alt.Chart(data.stocks.url).encode(color='symbol:N', x='date:T', y='price:Q')


line = base.mark_line().encode(
    )
marks = base.mark_point(fill='black', strokeWidth=0).encode(
    opacity=alt.condition(select, alt.value(1), alt.value(0)),
    ).add_params(select)

(
    line
    + marks
)

However, if I try to combine them the point selection stops working and only the line selection applies.

select = alt.selection_point(encodings=['x'], on='mouseover', nearest=True)
line_select = alt.selection_point(fields=['symbol'], nearest=True, on='mouseover')

base = alt.Chart(data.stocks.url).encode(color='symbol:N', x='date:T', y='price:Q')

selected_lines = base.mark_point(opacity=0).add_params(line_select)

line = base.mark_line().encode(
    strokeWidth=alt.condition(line_select, alt.value(3), alt.value(1))
    )
marks = base.mark_point(fill='black', strokeWidth=0).encode(
    opacity=alt.condition(select, alt.value(1), alt.value(0)),
    ).add_params(select)

(
    line
    + selected_lines 
    + marks
)