I think this is just way too individual to answer... each child is different, each family uses a slightly different homeschooling system, and you never know where the two will meet (or not).
My kids are 19.5 months apart. While I only think of my older child as "homeschooling" at this point, the younger one is very interested and involved. Who knows what we'll be doing next year?
The important thing is, they're both happy. They've got a great sibling relationship, their personalities mesh well and they love each other dearly. I think that all those things are just as important in terms of homeschooling as they are in terms of, say, going to the park. I don't know how much of it has to do with their age difference, their personalities, etc; I really don't worry about that too much. The kids are happy with one another as brother and sister, and I think that will continue no matter how homeschooling goes. Sure, I'm a bit paranoid about BooBah "passing" BeanBean in terms of academics (she is much more interested and motivated than he is at this point, for some reason...
) but I think that might be a concern for me even if the age difference was greater than it is.
This new baby will be born when BooBah is about 22 months old. Truth to tell, I've been much more concerned about how the loss of my milk would affect BooBah than how homeschooling might be adversely affected by the spacing between BooBah and NewBean.