I don't think you can merge two ids.
No need to delete one you don't want; just don't use it and it will become dormant.
Make sure all your devices are registered to the one ID that you decide to keep.
On devices using the other ID, you will need to unregister the other ID first.
Once a device is registered to one ID you won't be able to read books associated with the other ID.
I do not generally recommend DRM stripping, but this seems to be a case where its use is justified as the horrors of the Adobe DRM system effectively tricked you into your current situation.