On Elasticsearch 7.x the quorum is automatically configured so you should not hit any split brain issue.
On Elasticsearch 6.x you should not have the split brain issue if:
- you have only one master eligible node and the minimum master nodes is set to 1
- both nodes are master eligible and the minimum master nodes is set to 2
The problem is with 2 master eligible nodes the cluster will become read only as soon as one of the nodes is disconnected.
The problem with 1 master eligible node is the cluster will become read only as soon as the master nodes is disconnected.
Meaning you cannot perform a rolling restart and you'll always have downtime when in maintainance.
If you can afford a small additional instance, you could setup a 3 nodes cluster, where the third node is a master voting only node and the other 2 nodes are master eligible & data nodes. The master voting only node will not store data and participate only to the master election, allowing you to perform a rolling restart.