How many Audible accounts do you have?
If you have two Audible accounts, one option is to connect them to each other with Amazon Household.
https://www.amazon.com/myh/manage?
Once you set up your accounts to be part of the same "household", you can go into "Manage Your Content and Devices", then "Your Devices" and set each device to "Show [other account]'s content".
https://www.amazon.com/mn/dcw/myx.ht...home/devices/1
I have confirmed that immersion reading or Whispersync for Voice will work if you have an ebook on one account and the corresponding audiobook on the other, and have content from both accounts shared through the same Household.