Dog Soldier

Both are based on manga by the same author: Tetsuya Saruwatari. Both anime were made in the same period (1989-1990). They have main characters who look almost identical, as well as similar animation. They both have similar plot elements, since they focus on a battle between former friends/siblings. Also, they have a lot of action scenes. 

Both follow protagonists who are proficient with weapons as they try to complete a hard and risky mission, which turns out to be differently than the characters expected. They feature a lot of GAR action and violence, and some events also happen in South American forests. 

Similar lead characters, Suzaku = Dog soldier's protagonist & Lelouch = Dog Soldier's antagonist. Lead characters were childhood friends in both anime. Both involve military, warfare and deadly weapons. If you liked Code Geass, you will LOVE Dog Soldier!