Happy to report I got it working following your advice.
For future reference, this is what I did:
1. Put IB on 192.168.0.1, network mask 255.255.0.0. Set DHCP range in IB to 192.168.0.101-192.168.0.254
2. Add static DHCP route for USG in IB configuration, give it 192.168.1.1
3. Factory reset USG, connect to IB 1 gbps LAN port
4. Put USG in DMZ in IB config
5. Restart AP, adopt USG.
Now the Unifi wifi is working fine and giving devices IPs in the 192.168.1.x range, whereas the IB wifi is also operational and giving devices IP in 192.168.0.x range. Once I can confirm everything is 100% OK with the unifi wifi I will turn that one off.
Thanks for the help!!