To have multiple SLA's for a company you need to have multiple (Service) Contracts.
In a contract you can define what services that the customer have, and what SLA that cover the services.
So for example:
Company --> Contract 1 --> Service A, Service B --> DEFAULT SLA
Company --> Contract 2 --> Service X, Service Y --> GOLD SLA
ACME Inc --> SERVICE-CONTRACT-101 --> Hosted Exchange, DNS --> DEFAULT SLA --> SLA Target 1, SLA Target 2
ACME Inc --> SERVICE-CONTRACT-102 --> Backup service, Firewall --> GOLD SLA --> SLA Target 2, SLA Target 3, SLA Target 4
More info here :