You will need to create a bond (which requires at least one unused interface), then you will need to change sync to use the bond instead of whatever interface it's using now. Once you have the bond and the members are using it for sync, you can add the old interface you used for sync to the bond. Redundancy is then handled by the bond configuration, which is local to each member.
Note that sync traffic is always from the same port, IP, and MAC to the same port, IP, and MAC. LACP will always hash it to the same link, regardless of configuration. Round-robin transmit link selection is generally best for this, as it ensures all of the available sync links are actually tested.