Nothing more ridiculous than The Oracle Season/Age on the GBC.
You need to buy both games in order to carry over your progress, unlock new content and get the true ending.
That's pretty tame compared to most in this topic. Both games are pretty good and are completely unique from one another so it's worth playing both anyway. Now if something like Pokemon or Mega Man Battle Network tried to make you buy both games for the true ending, that'd be a different matter.