For terminology, I'm going to call your 2 clusters "source" and "replica". "Source" is the Scylla cluster that you want to backup, and "replica" is the cluster that you want to copy the data to. For this to work, you need a mapping of "source" node to exactly one "replica" node.
Ex:
Source cluster = 3 Nodes (source1, source2, source3)
Replica cluster = 3 Nodes (replica1, replica2, replica3)
Where we have a mapping:
source1 -> replica1
source2 -> replica2
source3 -> replica3
The end result of this process will be that the replica cluster tokens mirror those of the source cluster.
1. Start with the replica cluster shutdown. ($ sudo systemctl stop scylla-server)
2. On each node of the source cluster, run the following:
HOST_IP=`grep -e '^listen_address' /etc/scylla/scylla.yaml | awk '{ print $NF }'`
nodetool ring | grep $HOST_IP | awk '{print $NF ","}' | xargs | sed 's/,$//g'
This produces a comma-separated list of tokens for that particular node...
3. Take this list of tokens and plug it into the corresponding replica node's scylla.yaml file under the initial_token: property.
Now finish the normal restore procedure on the replica cluster (replacing the keyspace data w/ the back-upped data from the source cluster)
4. Start up the replica cluster, run nodetool repair, and once everything's up, you should be good to go...
Good luck!