When doing VPN monitoring between PAN firewalls, the tunnel interface does not have to have Ping enabled on the management interface profile (it doesn't even need a management interface profile).

You can also set the IP address on each tunnel interface to be a /32 (or just type the IP and don't set a subnet). Subnets are not important for the VPN monitoring if you don't mind the associated security rules looking little odd from the Zone point of view. You can select a /16 and then use the third octet to represent the local firewall and the fourth octet to represent the remote firewall.

Unless you use a /30, the rule will need to be SZ_VPN to SZ_Outside as the routing will think that the other IP is out of the normal default gateway interface.

E.g. 10.10.1.2 on one firewall and 10.10.2.1 on the second firewall.