Hi,
Configuring HA between two Balance 380’s with different hardware revisions is possible.
Keep in mind that you will get the best results when both Balance 380’s have the same hardware revisions.
I would strongly advise to replace the older Balance 380, since you want to be able to rely on HA.
Find more information about HA here: Configuring 1+1 Backup by High Availability (HA)