[e.k.]

Rarely I need this and due to another post in the forum I´ll describe my workaround.
I do not link the other post from another user due to inappropriate statements about mike´s support.

Assume/Prepare
- 2 tablets with identical libraries
- Tab1: set annotation layer 1
- Tab2: set annotation layer 2
- Make (different) annotations to a song (pdf), also at the same time (e.g. rehearsal) on different tablets.

How to merge annotations:
Tab1: embed annotations
Tab2: DO NOT embed annotations

Tab1: Server; Two-Way-Sync; uncheck all checkboxes except "sync annotations"; Promt user for decision
Tab2: Client; connect

Following will happen:
Tab1: Server ask for sync pdf: choose "mine"
Tab1: Server ask for sync annotations: choose "theirs"
Ready. Annotations are merged.

Repeat this steps everytime annotations are changed.

Maybe this will work vice-versa and I´m not shure if it works with same annotations layer on different tablets.
Didn´t investigate this deeper. Also, no idea about other consequences. 
As I wrote, it works for me.

As a hint:
Every embedding/sync the *.pdf file becomes noticeable larger (mike described the background sometimes ago in a post).
This is not really a "problem", I never work with originals, always with copies.

Maybe someone can use this (always make a backup before!)
e.k.

[Cajun_Organist]

Thank you for posting it this!  This is an important feature for me in my particular use case; however when following the steps you provided, on exiting annotation mode on the iPad embedding to the PDF,  I get the spinning circle “please wait”. It gets hung up, and the app frequently crashes;  then the devices become disconnected and quite difficult to reconnect, and the annotations are generally lost. Any suggestions? Thank you!.

[Zubersoft]

That usually points to a conflict between the PDF library and that particular PDF. If you extract the images from the PDF and rebuild it using a reliable tool like Adobe Acrobat, then you should have no issues embedding annotations in it. Email me at mike@zubersoft.com if you want to send me a PDF so I can rebuild it for you and you can let me know if it resolves the issue.

Mike