I am trying to add multiple select menus that contain different external data in both menus. But it seems you can only add one endpoint to 'Options load URL' under 'Select Menus'.
Is it possible to add multiple external select menus? I have been referring to https://api.slack.com/reference/block-kit/block-elements#external_select
The external select can only currently reference a single endpoint, however you can use multiple external selects in an app. Your endpoint will need to be able to disambiguate between which select it is and then respond with the appropriate data.
If you have one select for City, for example, and a second one for State, your endpoint might look for the action_id to know whether to query for city or state value.