Is a return ticket mandatory for travel to the UAE on a tourist visa? If yes, has it got to be on the same airline too?

Yes, it is mandatory for all travelers to the UAE to hold a return ticket from any airline, that is valid for travel within the window of visa validity. This is mandatory for all guests arriving into UAE except for resident or employment visa holders. The requirements are not exhaustive. Do check with the concerned government authorities of the respective countries prior to your travel for the latest updates and required documentation.