Hunter here from New Relic Support! I wanted to chime in on this thread since this question has been raised by a few and for posterity I’d like to answer here.
At this time it is not currently possible to merge two heroku add-on accounts into one account instance. With the New Relic Add-On with Heroku, each app that you’re wanting to monitor will create a new account within New Relic. That said, it is possible to move away from Heroku and open a New Relic direct account and this NR direct account would support connecting as many applications as you would like to ingest. To set this up, you’ll want to close out your Heroku Add-On account (see docs) and step through our sign up process to create a new account.
I certainly hope this helps offer some clarity but do let us in support know if you might have any follow up questions.