N.M.
My children are 21 months apart too--son first, then daughter (they're 6 and 8 now). When they got a little older it turned out great because they played with each other so well. But at first it was hard. It was so intense taking care of an infant and toddler, but the good news is that they keep growing and it does get better.
For me, a double stroller was essential. I picked a Maclaren side-by-side and loved it!
I often let my son sit beside me while I was nursing the baby, and we would read or look at a picture book together. The newborn needed nursing so often, and it helped him not feel neglected.
And when the baby slept, if my son was not also sleeping, it was good to give him some one-on-one time (rather than doing housework), because he really missed that.
My son came to see me and his new sister in the hospital on the first day, after I was in my own (shared) room and rested up a bit. He was so excited, and we took some special pictures and video. (Those are some of my favorite pictures.)
But I have to say, those first 2 years or so with two kids were the hardest for our marriage, and I didn't expect that. So don't forget to make a point of keeping your own relationship strong.
Congratulations on the coming baby!