Put together a well thought out "argument" about why you think an older child/two at a time/out of birth order would be a good thing.
Also address any issues you think it might raise and how you will deal with them.
If you want a list of issues to consider, here are some:
Local Support Network
Research you have done
Birth Order/Sibling interactions
Younger Children in the home
Put it on paper and have ready to discuss with your SW when they call/visit.
You know best how you think things will work out with your family. You just need to argue your case with your SW.