Try using CloudFront, it will eventually distribute your static content in all AWS regions (or not, you can tune this).
Reducing the load at your servers and lowering the response latency. Using this service you will save your servers resources and have time to migrate the static content into S3.
In addition, CloudFront setup is very straightforward.
In the other hand, if you are willing to persist on trying to use shared storage, EBS (Elastic Block Storage) cannot be mounted in more than one instance at the same time, then you cannot use it, but you still have at least 2 alternatives: