Two young children and their sleep - what to do?

We have two boys, a 3.5 year old and a 1.5 year old and their sleep has become a problem.

The boys go to bed at around 8-8.30pm fairly easily, although I think they've become too reliant on our being in the room. Right now, the boys share a room and my wife and I take turns to put the boys to bed separately (i.e. my wife will put our 3 year old in his bed, and I will take the 1 year old lying on our bed and then carry him in when he's asleep).

Then, most nights, the boys will wake (separately), with our 3 year old wanting to come in our bed and our 1 year old crying. For a while, we allowed the 3 year old to come in our bed to stop a fuss, but I think it's become a rod for our back, and we would give the 1 year old some milk and he will fall back to sleep.

Recently, that's not worked - we're getting tired of a 3rd person in our bed, and the 1 year old wants someone to stay with him until he falls asleep. Last night I was up from about 3am to 5.30am getting the two boys to bed, and I'm exhausted.

We've tried splitting the boys up (putting the 1 year old in our bedroom, or a spare bedroom), but that's not working either.
I think both boys are not good at self settling, and have become too used to us putting them to bed.

So, does anyone have advice? It's driving us a bit mad with our interrupted sleep, and each night feels like a battle.