Irish twins in poly relationship

Recently my husband and I have accepted another woman into our relationship. Her son will be a year this month, and our daughter will be a month on the 7th. He has talked about having kids with both of us but I intentionally want biological Irish twins as she wants to wait until her implant expires to have another. Is this a bad idea? Has anyone ever intentionally had Irish twins or had a step/half child 11 months older than their own at a young age?