I amended my 2019 return to add a late arriving k-1. Now I am amending my 2020 return to reflect this change. What additional documents do I need to attach to my return?

Do not include your old 1040 nor your revised 1040 because the Form 1040-X reflects any changes there and becomes your new tax return.

Include any new/corrected document only if it shows federal tax withheld, and include in order only those Sequence Numbered tax forms and Schedules that changed or were added due to your amendment.
Attachment Sequence Number is on all the forms and Schedules that come after 1040.
If any forms specify an attachment, put all those attachments at the end in the same order as the forms they go with.

If you are using TurboTax you have to do this by visual comparison. The program won't break out those pages for you.

@lrambabu1