Multi-disk per machine is achieved by starting multiple fdbserver processes, one per disk. What is the error message? Look in journalctl -u foundationdb and also look for Severity="40" events in the xml files in /var/log/foundationdb/*.xml
Q1: If you have a single machine, you can only have “single” replication mode (1 copy exists of the data).
Q2: If you have a single machine, and you configure 3 fdbserver processes, and select “configure double”, your cluster will fail, because you need at least 3 physical machines to enable that. If you did that earlier, I suggest you delete your test installation, data and reinstall (or add 2 additional machines to your cluster).