If both battlefields are a Dreaming Tree, me and my opponent both have 1 unit on 1 of the Dreaming Trees and no other units. I play a Cull the Weak. We both choose our units at the Dreaming Tree, do we both draw a card?

No, neither player draws a card from The Dreaming Tree in this scenario.

According to the official rulings for Cull the Weak and The Dreaming Tree:

  • Cull the Weak does not target units. It instructs each player to choose a unit they control as the spell resolves.
  • Because this choice happens during resolution rather than when the spell is played, it does not count as "choosing a friendly unit with a spell" in the targeting sense that The Dreaming Tree requires.
  • The Dreaming Tree only triggers when you actively target a friendly unit with a spell (such as using a buff or targeted removal on your own unit).

Since the choice made during the resolution of Cull the Weak is not considered targeting, the trigger condition for The Dreaming Tree is not met.