You can find some previous answers on how to throw components around in these two links:
I suppose in your case you could do something like:
html.Div(
html.Div([
dcc.Dropdown(id='the-first-one'),
dcc.Checkboxes('blah'),
dcc.Checkboxes('blah'),
dcc.Checkboxes('blah'),
]),
html.Div(
dcc.Dropdown(id='the-second-one'),
style={'display': 'inline-block'}
)
)