I have $161 platform credits on heroku from GitHub student pack to spend for one hobby dyno:
The Heroku offer is for one (1) free Hobby Dyno for two years, which will be applied on your invoice as a discount of $7 for a Hobby Dyno.
I already deployed one spring app and changed its dyno to hobby. My question is if there is a way I could deploy second Java app and set it to use the same dyno which is assigned for the first app?
Right now I have one app which has hobby dyno and one app which uses free dyno. I read on the internet and people say that there is only one app allowed for one dyno and Heroku will charge me for the second one but there is also this thread How many apps are allowed on a 'Hobby' Heroku plan? which says that I can have more than one app per hobby dyno.
So what's the truth?
No, you can't run multiple apps on a single dyno.
Heroku's model is to run one app per dyno (and often many dynos for a single app). The question you reference discussed the Hobby plan¹, asking how many dynos can be run per account. Keep in mind that Heroku's model is different from the shared or VPS hosting you might be used to. Dynos don't provide an OS that you can put apps onto; they run apps directly.
¹Technically I don't think this makes a ton of sense since accounts don't have these kinds of tiers. One account can run free dynos, hobby dynos, and professional dynos. But there is a difference between unverified accounts, which can run up to five free dynos, and verified accounts which can run many more.