Hmm. I’m not sure if you could do it by adding another zerotier network.
I think you could do something like this:
| LAN | Subnet | "DHCP" | pi | router |
|---------------+-----------------+--------------------+--------------+--------------|
| ZT | 192.168.12.0/21 | 192.168.12.9 - 254 | | |
| LAN1 | 192.168.12.0/22 | 192.168.13.2 - 254 | 192.168.12.1 | 192.168.13.1 |
| LAN2 | 192.168.12.0/22 | 192.168.14.2 - 254 | 192.168.12.2 | 192.168.14.1 |