we have a custom suite that till now had a drop-down /select that when submitted would post to an external API triggering a sync the list has been getting to long and there has been areuqest to have two different dropdowns or lists. how can have that only one dropdown is active at a time

Why two different multi-selects? what issue are you having with the current multi-select?

it is a single select but the list is getting to long

There are many ways to deal with that: • Define filters/criteria to dynamically populated your select field. (For example the use select filters and the select field auto-populated with the results (narrowing down the list)) • Create a JavaScript auto-complete input.