Why do you want to specifically make sure that the data is written to the other data center? If you’re concerned about an outage at the first data center, a single remote replica won’t save you - when the first data center goes down, you’ve lost a majority of your nodes so the remaining node won’t be able to serve traffic or recover on its own. In order to survive node failures, you need at least three nodes; to survive data center failures you need three data centers.
CockroachDB is designed for (roughly) symmetric network topologies. If two nodes are in the same datacenter and the third is remote, then the remote node is likely to fall behind and be less useful as a backup (or it may occasionally win an election and slow everything down).