The Union is way too outnumbered to win a straight up fight and simultaneously hold both victory points throughout the battle.
What I do as the Union is immediately pull back from the cemetery victory point and let the Rebs have it, concentrating all of my brigades on the other victory point. I also pull my cannon back and place them on the hill/farm south of the victory point where my forces are concentrated.
I fortify my brigades and concentrate fire on individual Reb brigades as the attack, turning them back one by one. As this goes on, I send arriving reinforcements to join my concentrated brigades. I just have all my brigades hold against the onslaught until the battle is almost over and at the last minute send several brigades in a sprint towards the cemetery and have them capture it just as the clock runs out.
You just have to play the clock because there is no way to defend both points.
