have you tried balling rolling? I know it sounds really obvious but Rachel adores it and has done since she could sit up on her own. Of course I had to wait until I have someone else to play with but you have DS1 who can be opposite you and DS2 and you roll the ball between you. Your older boy can be as adventurous as he wants and can do some ball skills which will amuse your younger son, and it teaches your younger son about rolling and catching.
another thing could be building towers - so your older son builds with blocks or cups or anything and then he asks your younger son to knock them down. I am guessing that as boys they probably love the idea of destruction and so it should appeal to both.
hth