Adding both CNAME and TXT DNS records for one subdomain

You can't. As RFC1034 says in s3.6.2,

If a CNAME RR is present at a node, no other data should be present

If you want a TXT record for (say) www.example.com, you can't have a CNAME for www.example.com, and will have to find another way to achieve what you want. This may mean monitoring example.herokuapp.com yourself, and when the IP address changes, updating your own A records for www.example.com.