Here's how it works. Suppose your domain is "example.com" and your MX record is "mail.example.com". You would add two more MX records as follows:
example.com. MX mail.example.com. Priority: 10
example.com. MX mx-b1.sixwishlist.com. Priority: 50
example.com. MX mx-b2.sixwishlist.com. Priority: 100
That's all there is to it!