A high-available set up is a 6-node Dgraph cluster (3 Zeros, 3 Alpha replicas.
You can set --replicas=3 and run two Alphas, in which case the two Alphas would be replicated. But 2 Alphas is not a high available set up. If one of the Alphas stopped, then there’s no longer majority consensus in the replication group, and txns will not run without a majority of the cluster being up.
And, for your question about running 1 Zero and 1 Alpha on one node and the other Alpha on another node, that’s OK. Each Alpha should be on a different node so they’re not sharing machine resources.