Hi @hp_inkjet,
Interesting case 
This might complete BS, but for what it’s worth, some thoughts:
I guess you would like to run two different instances of Zerotier on two different switch, right? In the L2 scenario, if you would define the two remote connections as two separate zerotier networks, they should not create loops on the network. If you then have STP running, in theory it should disable packets from using one of the two links, no? In that case there’s no need for zerotier to handle anything…
L3 case looks tricky…
How fast should the failover be?
Maybe some scripts can help you out… (Maybe even in combination with the zerotier API)
Kind regards,
Timmmy