EBS can be attached only to one EC2 Instance at a time but recently EBS MultiAttach was introduced.

It has some limitations:

only with provisioned SSD volumes

only on instances based on Nitro

still only on same AZ

Multi-Attach do not support I/O fencing so it is necessary to provide a write ordering between instances to prevent data corruption.

For Multi-Attached EBS choosing a Clustered File System ( like GF32) is recommended