There was a topic on this last year Specify Botany Webinar: Multiple Specimens on a Sheet, perhaps this might be helpful? One approach to having multi-species collection objects is to figure out how to implement multiple determinations, but another would be to catalogue the truly different specimens as separate collection objects mounted on the same sheet.
*edit: this also brings up something else I’ve been really wanting in specify… an automated way to split and merge lots. Currently, if you find a lot that has more than one species, it’s quite hard to represent the relationship between the original lot and the new one without a lot of manual explanation. It would significantly speed up the process of dealing with stuff like this if there were a plugin/script that could auto-add in the correct relationships and stock text of a collection’s choosing when splitting or merging CO records, especially those that are in the same COG or container.