i assume you are following the docs and using the site prefix before the custom field?
Multiple Site Manager and orderby=
The orderby= parameter can accept a site short-name in the namespace.
orderby=“default_site:body|second_site:summary”
When ordering by multiple custom fields from multiple Sites, one must remember that entries from another site will have no data for that field, and the entries will be ordered as such. This results in ordering entries by Site and then Custom Field(s):
orderby=“default_site:body|second_site:summary”
Will result in:
Default Site - Entry One - Albert
Default Site - Entry Two - Bobby
Second Site - Entry One - Alligator
Second Site - Entry Two - Buffalo
If you have multiple Sites where each site has a custom field with the same, exact short name, then you can specify that short name (without the site specified) and ExpressionEngine will treat those two fields as the same value and be able to order them as if they were the same field: