Me and exp split several months ago, we already have ds who is nearly 2. The split was amicable, we still speak often and occasionally have days out with all of us together and it's not awkward. We are good friends.
I'd like ds to have a sibling with a fairly smallish age gap between them
My reasoning is
1: I know ex is a good dad, he has 50% custody of ds and is really good with him, always has been very hands on and involved.
2: if I waited to meet someone else down the line there wouldn't be the smallish age gap I'd like and there's also a possibility they could switch once baby is born or once I'm pregnant and become an uninterested waste of space. I'd feel so bad for the second child.
3: I think it would be easier in someways for them to have the same father. There's less likely to be resenment and jelousy between them.
4: I already know we produce very cute babies 
kidding!
Now I its uh a bit unconventional and i know things can change, I could still meet someone and have number 3 with them etc etc etc I'm not discounting all these things but as far as things go apart from the possibility of all that is this a stupid idea and why?
On a side note, I work and can afford to have number 2. I'm not expecting anyone to "pay" for my children. I know most of you will say this is irrelevant and your right but I also know there will be people at home who would otherwise assume I'm not working and not going to be.
I know it will be more difficult than it was with ds as exp would be around but not as much as he was with ds.