Most obstacles require both brothers. For example, the older brother might need to distract a guard while the younger brother sneaks past, or they may need to swing together on a rope.
Controlled with the Left Analog Stick and Left Trigger (L2/LT) . He is stronger and can pull heavy levers or boost the younger brother to higher ledges.
The younger brother is afraid of water. When crossing rivers, have the older brother swim first; the younger brother must grab onto his back using his interaction trigger (RT/R2) to be ferried across. 100% Achievement Guide (Missable Secrets)