You are correct that there isn’t a specific trigger for when a multi-select field has two or more choices selected. However, you can build up to this situation.
Create a formula field that counts how many items are selected in the multi-select field. The COUNT functions don’t work on multi-select fields, so you have to use a workaround by counting the commas in the field value.
Create your automation with a “when record meets conditions” trigger. Have the condition be based on the value of the formula field.
Here are a couple of cautions:
This method assumes that you do not have any commas in your multi-select choices. If you have a comma in a multi-select choice, this method will not work.
This method works best if the field value is being set with a form or some other method that sets all values of the multi-select at once. If someone manually sets the values one at a time, the automation may trigger before the person is done selecting all the choices and you may get unexpected results.