Rainbow,
Huge sympathy - been there, done that. DS1 didn't sleep through the night until he was three. I had to sit with him (or lie down with him) til he dropped off, then I would creep out, and between 10 -1 he would wake again and I would go in his bed with him. I had tried CC when he was younger but just didn't have the strength to do it (in my defence, I was suffering from severe PND and just getting through the day was an effort).
When he was three I had a kind of revelation and thought "NO..I cannot go on like this." I got very tough - left him to cry, had a star chart/reward scheme when he did settle down by himself, and - most important - when he woke in the night I would not get in bed with him, no matter how exhausted I was. At three I could reason with him - I would say "no, you're a big boy now and Mummy needs to sleep in her own bed, and you need to sleep in yours." It was hard, but it worked. We had subsequent "hiccups", like when we moved house when he was 4, and I just had to shut him in his room and leave him to cry - but, touch wood, he's a good sleeper now.
With DS2 I was tougher from the outset - CC from 8 months, and since 12 months he has slept through virtually every night. It has made this experience of parenting a much more positive one - you can cope so much better if you've had an unbroken night's sleep. With DS1 I was literally like a zombie much of the time - I don't know how I did it.
As others have suggested - can you put the eldest and the youngest in another room, even temporarily? You really just have to grit your teeth and go for it - stick to a rigid bedtime routine and you definitely need some way of keeping him in his room, we had a stairgate.
I do know how hard it is, believe me (and as the proud owner of a DH who never - NOT ONCE - got up in the night with our eldest, I can sympathise with that too). Good Luck.