How to create a multiple select dropdown in Trello
Amazing Fields for Trello lets you create dropdown fields that allow for multiple selections
See multi-select dropdown fields in action: Create a board from the Amazing Fields Feature Template.
Multi-select Dropdown Setup
Add a basic "Dropdown Field" under "Basic Fields" in the add field menu.
This will create a dropdown with a standard setup including only allowing a single section. To change it to support multiple selection, go to the "Advanced" section and choose to allow selecting multiple items. (note: this is a supporter-only function so you may need to become an Amazing Fields suppporter first)
Once you select allowing multiple, you can choose how you want those multiple items to show up on the card. You can have either a single "badge" with comma separated items or you can choose to have multiple individual badges.
Advanced Dropdown Settings
There are a tremendous number of ways to configure and setup your dropdowns. The best way to see what is possible is to experiment with all the different options.
If you have a specific request feel free to contact me through the support contacts and I will add another example.