Felidar Sovereign

• WOTC Official Reminder Textofficial rules
  Lifelink — Damage dealt by this creature also causes you to gain that much life.
• WOTC Official Reminder Textofficial rules
  Vigilance — Attacking doesn't cause this creature to tap.

• WWizards of the Coast2015-08-25
  Official Ruling
  Felidar Sovereign's triggered ability checks to see if you have 40 or more life as your upkeep begins. If you don't, the ability won't trigger at all. If you do, the ability will check again as it tries to resolve. If you don't have 40 or more life at that time, the ability won't do anything.
• WWizards of the Coast2015-08-25
  Official Ruling
  In a Two-Headed Giant game, your life total is the same as your team's life total. As such, the ability triggers if your team has 40 or more life.

Rulings via Scryfall