It's theoretically possible to set up the display values manually, but whether you want to bother probably depends how many combinations you've got. You could possibly write some Python to update the field properties as well; I'm sure there must be a theoretical limit on the number of CV domain pairs on a field in AGOL however?
One thing I do for domain value readability sometimes is to use a longer value, for example in your case I might use "EMERGING_SITE" as a code rather than "sites".
Another alternative - if you've only got three or four options in your select_multiple - is to delete the field and replace it with three or four yes/no fields. This gets ugly fast as you have more options however.