With CARP you need a minimum of 3 IPs per interface. One for the first box, one for the second box, and then one that is shared. That last one is the one that would be used as your gateway on the LAN side and is the one that would be used by your clients for presence on the WAN side. If you wanted to use different WAN IPs for different subnets then you'd need an additional WAN IP for each subnet (so 3 IPs for 1 subnet, 4 IPs for 2 subnets, 5 IPS for 3 subnets, and so on).