Your worry is natural, but you will be fine!
Put simply, you have no choice so you will just find a way to make it work.
Imagine first time mums wondering how on earth they will give birth… equally many wonder how it go to two or three.
The world is full of strong mums who’s done it and survived.
Your firstborn is still small, so it will be hard in the beginning.
What will most probably happen is that you will initially mainly take care on the newborn and you DH will be with the eldest.
Divide and conquer is quite a good strategy.
You will find a way that works for you all and you will soon forget what it was even like to only have one child.