The way I see it ... certain damselfish are by nature aggressive. It's too bad that many books, most every lfs recommends using them for "starter" fish. Then you're stuck with them when you want to add other non aggressive fish.
Catching them can be tricky in a tank with live rock.
Chances are you'll have some aggessive behavior, but as tk2020 mentioned, there are exceptions.
Our yellowtail damsel is quite peaceful and knows it's place in the "pecking order".
So Catherine ... to address you question, depending on the size and species of fish you want to add, and that you have 20 gallon tank, I would say most likely the 2 damsels will attack it. The outcome is usually not pretty, and it can be a real shame to lose a new fish.
I'd suggest removing them - then you need not worry about it - and move on with stocking your tank with other fish.
Don't blame the damsel for it's aggressive nature or if it attacks another fish.
It's just doing what damsels do -
Brian