I have this problem!!

I find playing a baby music CD (ie - 'music for dreaming') when I'm putting DS2 down helps to drown out the DS1 noise. And definitely have ds1 engaged in an activity when you put ds2 down - if possible!!
I also put my DS2 on his tummy (controversial, I know). Only way I could get either of my boys to self settle and for me to get any more than an hours sleep at a time! I love the fact that they cant watch you walk out of the room! (However, I wouldn't recommend it unless you have an organic mattress though as the standard ones are treated with flame retardant chemicals which have been linked to SIDS).