We listen to two different Audible books on two different Amazon Alexa devices in our house. One we listen to in our lounge room (connected via Bluetooth, using Alexa as a Bluetooth speaker), the other in our bedroom (not using Bluetooth). We find that if we haven't listened to an Audible book on our lounge device for a day or two, and then we do, we lose the syncing spot for the (different) Audible book in our bedroom, and have to search for where we were previously (rather than just asking "Alexa, resume Audible"). Painful!
The only work around I have found is before we play the second book in our lounge, we have to select "sync across devices" via the Audible app in order not to lose our place in the book in our bedroom. This has to be done before we play the (second) book in our lounge.
Any tips would be gratefully received. :)