You cannot, unless you use the same account. Email authentication configuration is done per account basis, not per domain basis. This means each unique Sendgrid account must go through the authentication setup in order to send DMARC authenticated mail. This is due to the way Sendgrid handles their labeling for your return-path subdomain, which includes the account’s unique ID in it. For example (em234.example.org).
However, there is one important configuration detail you need to be aware of. Since the two accounts will be using separate authentication configuration, including a unique DKIM signature for each, they both default to the same selector (s1). I strongly recommend you choose the custom selector option which will allow you to choose a selector of 3 alpha/numeric long string. This is because a DKIM public key cannot share its selector with another.
I hope this helps.