Is it possible to join tables stored in different entities on the cloud with a unique primary key?
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Typically (although you may have a different specific use-case) you join two database tables because they're related AND because you want to avoid the overhead of multiple round-trips between Appian and the database (so you can get your related data from those two tables in one call)
Typically (but same caveat as above!) those tables are related parent-child (that is one-to-many). The act of joining those tables (to create a VIEW) means denormalising the relationship between them. Given that one 'parent' will have many 'children' you would see a list of 'children' many of which would have the same 'parent'. So you would be safe in using the child's Primary Key as the unique attribute to identify each row.
Once you have a VIEW you can then use the tooling in Appian to create a single CDT from that VIEW, hence "combining" two CDTs.If you had other related tables you simply look for the entity at the bottom of any relationship hierarchy and choose that Primary Key as your unique attribute.