I can't find anything about this particular issue after searching a while so I'm at the point where I think I might be going about it the wrong way. My understanding of DNS is not great so I might just be missing something obvious.
Say I have a domain tomato.com and I have two Heroku apps, one staging and one production, and I've added the Mailgun add-on to both of them. In the Heroku CLI, I added tomato.com as a domain to my production app (Heroku defaults to ALIAS or ANAME), and staging.tomato.com to my staging app (Heroku defaults to CNAME). In Godaddy, I've added a CNAME DNS entry with the name staging and the "data" the DNS Target provided by Heroku. All of these seem to be working correctly in the browser.
In Godaddy, I was able to add TXT DNS entires for SPF and DKIM with the values provided by the Mailgun addon in my production app for tomato.com, but when I try to add a TXT entry with the name staging and the data v=spf1 include:mailgun.org ~all, I get an error: Record name staging conflicts with another record, I'm assuming due to the CNAME record I added for staging in order to get that subdomain to redirect to the staging heroku app.
How can I add the Mailgun DNS entries required for verification of my staging app if I can only make one entry of any type that uses the staging name?