Thanks Gane, I must have missed that option. I'll give that a try!
In the meantime my workaround was to install Sync on a second device, with the alternate network set to a higher metric. Their destinations are each the same shared storage. I think it's a little awkward because I noticed they would send data to each other, so I ended up disabling LAN sync. Not sure that was the best workaround so I'll try a link aggregation when I get the chance.
Thanks again!