Hi @humpty Dumpty ,
Since you have to two different tables, I don't believe you can utilize Azure SQL to update Azure Search Index as you can't use views in change tracking.
What I would suggest is when you app makes the changes to Guest or Travel DBs, pull the relevant information from the two database into your DTO and update/insert the index document. There are different methods you can use as a trigger such as QueueTrigger for a webjob or HTTP Trigger for an Azure Function.
Regards,
Ryan