How to update 2 CDT's with each other values in a process model

I am new to Appian. Still learning the basics.

I have two tables

1. Attachments - for adding attachment relate details like fileId,fileName, fileExt, fileSize etc and

2. AttachmentDetails - for description, comments etc

Below is the requirement -

When i upload a document I save them to the attachment table with attachmentDetails primary key (attachmentDetailsId)

After the documents are saved in the attachment table I need to save the attachment tables primary key (attachmentId) to the attachmentDetails table

The problem is i am not sure how to update when there are multiple attachments.

For Example - If i  upload 2 attachments and update attachmentDetails

Attachments Table

 Attachment Details Table

I will first save the 2 attachmentDetails (array) to the db first. (at this point attachmentId will be null and I will have the attachmentDetailsId's)

Before saving the attachments to the table i need to map each attachment with attachmentDetailsId. How do i map attachmentDetailsId-1  with first attachment and  attachmentDetailsId-2 with second attachment?

After I map the above I will save them to the attachments table (at this point I will have the attachmentId's)

How do i now map attachmentId-1 to the attachmentDetailsId-1 and How do i now map attachmentId-2 to the attachmentDetailsId-2?