Cosmos DB where condition by external document - sql

Question

I have a following structure of document (omiting all with underscore prefix like _self)
{
"id": "c5055e2b-efb2-4c86-907d-a0beb1dca4dc",
"Name": "John Johnson",
"partitionKey": "0ecdb989-01c6-4f11-9fd2-3e1dcc1c8cb9",
"FKToBeDeleted": "FK_c5055e2b-efb2-4c86-907d-a0beb1dca4dc_ToBeDeleted",
}
And as You can see there is a field named FKToBeDeleted and I use this to mark document, but it has to be as a reference, because in my app may occure kind of database concurrency, because 1st app can GET document, process it, and 2nd app can update document during processing and 1st one will not see any changes, because downloading again huge document and updating it is RU consuming, so I wanted to reduce the cost. Going further, I created a document for this.
{
"id": "FK_c5055e2b-efb2-4c86-907d-a0beb1dca4dc_ToBeDeleted",
"partitionKey": "0ecdb989-01c6-4f11-9fd2-3e1dcc1c8cb9",
"ToBeDeleted": false,
}
And now there is a problem, because my front-end app does not want to display any ToBeDeleted documents. This kinda cheats the user, because I just mark it as deleted but later delete the document.
Now the question is how the SQL query should look like? Previously it was like the following query, because r.ToBeDeleted was boolean.
SELECT r.id, r.Name, r.AddedAt, r._ts
FROM ROOT r
WHERE
(NOT(r.ToBeDeleted))
ORDER BY r.AddedAt desc
Now FKToBeDeleted is only a reference to another document, but the ID is in r.FKToBeDeleted, so I tried some nested SELECT but it didn't work.
Any suggestions what is the right way to achieve that?
EDIT (clarification)
Let's have a following situation.
There are two apps (you can also treat them as threads) which uses the same Cosmos DB instance.
STEP 1 - is a moment of start of processing some data, but database document is needed, so it gets that and on the right side you can see current document (but in fact only ToBeDeleted is interesting here).
STEP 2 - is a moment, when user wants to remove this processed item, because he is no longer interested of its results, but database document is also required here, so again there is a GET.
STEP 3 - is a moment, when job of soft delete is done and there is a need to update database document, and the field is set to true.
STEP 4 - is a moment, when processing is over of common flow and at the end there is update of the document. BUT, Application 2 downloaded it before STEP 3, and it's overriding things that Application 1 did, which is bad.
So I made a solution for that.
As you can see, the steps are the same, but instead of updating the same document, I update a referenced document, so I don't have a problem with overriding data.
Now, the problem is how to make a SQL query to join two documents, so the FK_1 id will be replaced of the value of ToBeDeleted field in another document.

Answer 1

According to this article there is no possibility to join two documents, which of course does not help me at all, yet closes the topic.
JOIN keyword exists in the language, but it is used to “unfold” nested containers, there is no way to join different documents.

Answer 2

Perhaps, you can use subquery instead of JOIN.
https://learn.microsoft.com/en-us/azure/cosmos-db/sql-query-subquery#mimic-join-with-external-reference-data