Recurrent auto-sync of assets from one S3 bucket to another in separate account

Question

The problem:
I have an old S3 bucket: bucket A and a new S3 bucket: bucket B. These buckets are in separate accounts. Up until now, I have been serving assets from bucket A. Moving forward, I want to serve assets from bucket B. I must still support pushing to bucket A. However, those assets pushed to bucket A must be retrievable from bucket B.

Possible solutions:
On every new push to bucket A (PutObject), I must sync that object from bucket A to bucket B. As I understand it, there are two ways to achieve this:

1. Using AWS Lambda with Amazon S3
2. Using DataSync <-- preferred solution

Issue with solution 2:
I have a feeling the path using DataSync will be less complex. However, it's not clear to me how to accomplish this, or if it is even possible. The examples I see in the documentation (granted there is a lot to sift through) are not quite the same as this use-case. In the console, it does not seem to allow a task across multiple AWS accounts.

The disconnect I'm seeing here is, the documentation implies it is possible. However, when you navigate to DataSync Locations in the AWS Console, there is only the option to add locations in your AWS accounts S3 bucket list.