Interesting question, i actually need the answer too soon. I think, that this will workout.
First, forget the domains for a moment. We are now in the "Bindings". In "Port", you see the usual ports like "SMTP SSL", "POP SSL" and so on. All SSL-Ports are bound to a certificate. The Ports have a binding to an dedicated IP address.
Now we can add additional set of Ports named e.g. "POP SSL customer 1", "SMTP SSL customer 1" and so on. These Ports must be bound to a second certificate. Finally these ports needs to have a binding to a new IP-address, which can be added under Bindings -> IP Addresses. Each IP-address needs a valid host name, which can be added under "Bindings -> Hostnames".
I didn't test this scenario, but i'm pretty sure, that this will work.
If you add a new domain, you can choose between the different IP-addresses for the outbound IPV4.