oooh congratulations!!! 19 months between mine and I have to say - it's alot of fun and much easier than I thought it would be. I would say that as long as the older one is walking, you'll be fine. I absolutely love it and I was terrified!
It can take longer to leave the house BUT having said that, I manage to get them up and sorted and me up and sorted and out of the house in an hour (from all of us being asleep) from the word go, so it can't be that bad!
Tips:
Have nappy changing stuff and spare clothes and PJs for both on every floor of your house (just makes life easier).
Get clothes for next day and buggy loaded up and snacks / lunch for older ones etc sorted the night before.
Turn on the TV if you need to keep them occupied while you cook dinner or need to do something else quickly - don't use it all the time as it doesn't work so well if you do!!
Get a sling that you can bend over hands free with (e.g. wrap sling) - it will give you options for bed time / in the park / shopping / bath time etc
Accept any help on offer and if nursery or childminder is an option for any time, go for it so you can catch up with yourself / lie of sofa / enjoy new baby
Don't panic - the worse that can happen is they all scream at once hungry, tired and covered in poo and sick. That's not life threatening. Just prioritise and methodically work your way through to getting them clean, fed and sorted again, ignoring the noise and tantrums and sure enough, before you know it, all is well again!