This is correct @corrado.steri the feature to allow this to work with only one side having multiple links is not available yet. In the ticket you linked to I mention following the progress in the dev-multipath branch. This feature is still in progress but not too far off.
The best place to ask about experimental multipath features on bleeding-edge branches is on GitHub, not this forum.
Additionally, not all multipath modes are ready for production yet but active-backup, balance-rr, and balance-xor are pretty stable at this point in the dev-multipath branch.
If you’re still having trouble getting it to work I would suggest making a ticket on our main repo and tell us what you’ve tried and what your environment looks like. Next week I can take a look to see what may be wrong and try to assist.
The new docs site at docs.zerotier.com will be updated as the dev-multipath branch matures.
Good luck and hope to hear back.