Can the USB drive containing Tails OS be shared with other files?

1. Can a USB stick be bootable while storing other files?
2. Does Tails have to be on USB?

Can a USB stick be bootable while storing other files?

yes !! you can put files to a bootable pendrive -your question should be "will it be still be bootable by system if i put other non-related files/folders in it ?" and another yes for this question too -->make sure you make a new folder and put all those non-related files in it !!

Does Tails have to be on USB?

Hardware requirements: A USB stick of 8 GB minimum or a recordable DVD. All the data on this USB stick or DVD will be lost when installing Tails. The ability to start from a USB stick or a DVD reader.