Drupal Search API - multi index View search on two fields

We have a D7 site using Search API with two indexes (one for nodes, one for comments on those nodes).

We are displaying the contents of in the indexes combined in a view using https://www.drupal.org/project/search_api_multi which lets us show both indexes in a single view, one entity (either a node or a comment) per row.

We have one remaining issue which is to sort the results of the view by entity creation date. As these are different entities with separately indexed creation dates we need to combine the entity creation date fields for each in order to sort. Standard contrib modules for doing this such as Views PHP Sort don't appear to work, most likely because of an incompatibility with Search API. I'm expecting that a custom module will need to be written for this.