Multiple ISPs should be handled with multiple separate external interfaces. These can be tagged subinterfaces of a physical interface, or they can be multiple physical interfaces. You would then configure ISP Redundancy in the cluster object.
As for multiple public blocks from one ISP, how is that supposed to work? Are both of these networks on a single broadcast domain?
If they're both on one broadcast domain and you don't need the firewall itself to initiate new connections out from one of the IPs, you can always just add a proxy ARP statement for it. That will get the traffic from the broadcast domain to the firewall, where you can apply NAT or whatever.