I have 2 domains, the second one being an alias for the first. I know I would need to purchase 2 store and forwards to make incoming work. My question is would I then have to purchase 2 Outbound SMTP Relay services or can only 1 relay email for both domains.

Outbound SMTP relay service is also domain name based. You will need to sign up for the 2 domain names separately. You may sign up for the service here.