To obtain a 2nd builder, you must go to the in-game shop, which can be reached by tapping on the "+" button on the right side of your gem total or by selecting "shop" in the 2nd part of the menu. You then tap on the VIP Pass and pay the amount. You will then receive an in-game notification that delivers the 2nd builder.

You gain access to a second builder when you purchase the VIP gaming pass from the shop. Click the menu button in the bottom right of your game screen, and tap the shop icon. A VIP pass gets you a second builder, daily gems, daily loot tickets, and more.