I had my second just before my oldest (with ASD) turned 3. When she was convinced he was just 2 and we where putting a lot down to extreme toddler behaviour, but by the time she was born it was clear there was more going on with him, and he got his diagnosis 18 months after that.
If I'd have known at the time how hard the first two years would be, I don't think I would have had a second, but now we've been through hell and are starting to come out the other side I'm actually kind of pleased I didn't know. There coming up 6 and 3 now and thing's are getting easier, they get on really well, my youngest is a delight and I think my eldest is really benefiting from having a sibling.
If I'm honest there were compromise along the way and at times the both received less attention then would have been ideal. I think you need to think through what changes or compromise you would need to make, and then decide if the benifits of another child outweighs it. For us the main change was that I left work, so there's more of me to go around at home, and I also used child care to give each 1:1 time and give me occasional breaks. I kept eldest in nursery part time when baby was born, and started youngest at 1. As a result our finances have taken a nosedive, but we get by with help from DLA and carers allowance.
Youngest spent most of her first year in a sling, as she wasn't safe if I put her down, and was BLW on toast and DS's 'safe' foods, as I didn't have the time to make separate meals or spoon-feed. Eldest watched an astronomical about of TV and sometimes has to wait for attention, it doesn't happen often but sometimes I have just had to leave him to kick and scream in a safe place, and get the baby to a separate safe place if he has a violent meltdown, where as I'd have stayed with him (despite been hit) to help him calm down when he was an only.