I have two domains that both need to use one server that has a dynamic ip address for email.

domain.com: Static IP, A record pointing to the static IP, needs to host e-mail for @domain.com but use domain.biz as mail server.

domain.biz: Dynamic IP, using DYNDNS to update IP address, hosts e-mail for @domain.biz

The way I have it working now, is not supposed to be allowed (Because of an MX record pointing to a CNAME):

for domain.com:
I have a CNAME for mail.domain.com --> domain.biz
I have a MX record with priority 0 --> mail.domain.com

for domain.biz 
I have an mx record with priority 10 --> domain.biz

Should I fix this by having the MX record for domain.com point to domain.biz?