Hi. first big congratulations. And second don't panic. You've got 8 months or so to get used to this. I was pregnant with twins second time round and I did worry about how I was going to cope with the older one and potty training. I decided I was going to not try to make her do anything before she was ready because life was going to be pretty confusing for her when her sisters came anyway. So she was nearly three when she was potty trained but at least it wasn't stressful for us.
In terms of the birth, my second birth with twins was so so much better than the first (normal delivery). I hope it will be much better for you too. Talk to your midwife early on about your worries.
I hope your pregnancy goes really well. It my experience the second one went very fast compared to the first because I was so busy and not thinking so much about being pregnant. It's normal to worry but it's going to be ok.