One of th requirement of Multisite indexer cluster with SmartStore is
Site locations host two object stores in an active-active replicated relationship. Depending on the deployment type, the set of cluster peer nodes can be sending data to one or both object stores.
To fulfill this requirement, is there some configuration I have to do in Splunk or in S3 bucket ?
Just like that. In cluster you specify only one path for remote storage/volume and then use it on individual indexes as volume.
https://conf.splunk.com/files/2019/slides/FN1435.pdf page 35 is configuration example.