heroku subdomains on different apps?
Each subdomain should be a CNAME to the Heroku app you want.
In your DNS provider (something like GoDaddy, DNSimple, et. al), setup like so:
subdomain | record type | end point
www | CNAME | main-app.heroku.com
game | CNAME | other-app.heroku.com
Make sure you add the matching domain to the corresponding app with $ heroku domains:add