For multipathing, you will want to check out installing the package multipath-tools:

apt update
apt install multipath-tools

Once you have the package installed, you will want to edit the configuration file located here:

nano /etc/multipath.conf

You can create a general configuration using something like the following example:

defaults {
    user_friendly_names yes
    find_multipaths yes
}

blacklist {
    devnode "^sda"
}

devices {
    device {
        vendor "Synology"
        product ".*"
        path_grouping_policy group_by_prio
        path_checker tur
        rr_weight uniform
        no_path_retry queue
        failback immediate
    }
}

After you create the configuration file, you will need to restart the multipathing configuration:

systemctl restart multipathd

Discover your iSCSI target by IP addresses:

iscsiadm -m discovery -t sendtargets -p <target_IP>

Log into each target session:

iscsiadm -m node --login

After logging into the IP addresses with the above command, make sure multipath devices are created using the command:

multipath -ll

You should see a multipath device (will see something like "mpatha", etc) that combines both iSCSI paths.

Take a look at this official documentation here as well from Proxmox: Multipath - Proxmox VE.

create storage failed: command '/sbin/pvs --separator : --noheadings --units k --unbuffered --nosuffix --options pv_name,pv_size,vg_name,pv_uuid /dev/disk/by-id/scsi-3600140533763436dbfedd4c20d9b20d6' failed: exit code 5 (500)

How do I make use of my two iSCSI connectors to my Synology LUN properly?