I have been planning on getting another bird now for a while but I've been thinking aswell if they wont kill each other, and will they need to have seperate cages
Personally I would err on the side of caution and have separate housing for each bird. There is never a guarantee that two birds will get along together, whether they are the same species or not. Plus of course if they are the same species and of opposite genders there's always the possibility of unwanted breeding taking place. I've had birds of the same species that I would not dream of caging together, and totally different species who were the very best of friends and lived together very happily - you just never can tell who will be compatible and who will not, and if they're not it can be disastrous. I think you should definitely make arrangements for your birds to have separate quarters, for safety's sake.
Superb advice above! Are you considering a second quaker or other species? Size differential definitely argues in favor of separate cages. Multiple parrots often compatible on neutral territory such as playstand or remote perch. Another potential issue is bonding together, excluding you as "third wheel." Does not always happen, careful nurturing and time sharing may alleviate!!