Widdley,
My little girl was only 3months old when we conceived our second little bubba. We were trying to conceive but thought it would take a little longer as I was still B/F, so we were aiming for conception on month 3-4, not really the 1st month. But we were happy & blessed with what we now know is our little man.
It was hard when DD awoke a couple of times during the night, but I honestly think that it would be easier while they are younger than older when they are not crawling or running about, as they still stay where you leave them, they still have 2-5 sleeps a day so you can rest up too. And as I'm hoping the closer than are when they hit the terrible 2's, we will hopefully only have to go through them once not twice, well I have been told this anyway, from another mother who has now got a 3&4 year old. Plus there might be a better chance of them getting along more when they are alot older, as they will grow up together, again just another thing we are hoping for.
What ever you decide I hope you get what you want.
Sorry if I couldn't help you out.