I really feel you on this, two under two is such a big transition and it is completely okay to feel overwhelmed before the second one even arrives. The one thing that helped me most was setting up little "safe zones" for my toddler, like a play yard or a gated area, so I could nurse or settle the newborn without worrying where my older one was running off to. Also, do not be shy to ask your confinement lady or whoever is helping you to take the baby while you give your toddler some one-on-one time, even just 15 minutes makes a difference for the older one emotionally. Stock up on easy snacks and activities at toddler height so your 13 month old can self-occupy a little, and please remember to eat and rest when you can because you cannot pour from an empty cup lah. You are already doing amazing just by preparing ahead, hang in there mama!