As you might know, once you finish a three-star unit, the store can no longer roll this unit. This means that in order to get two three-star units, you need to exploit this mechanic:
You need to put your three-star unit on the bench and roll afterward. This will enable to get find this unit in the shop again.
Getting two three-star units is pretty hard to do. This game was a "special rules game" in which the players couldn't level-up, which meant that they had to spend all of their gold on shop rerolls.
But even in a real game, if you get lucky and you get a quick important three-star in your lineup (Terror, Razor, Shadow Fiend, etc.), it might be worth it to at least try to get a three-star + a two-star, which is much more realistic especially with the help of Io.