Ringnecks are birds that flock together in the wild and depend on the flock for moral support, protection and company. They will groom one another and roost together, keeping each other company during the times when you're not available. They'll even mate, which defuses some of the possible dependence on you for fulfilling 'that' part of their lives. Don't provide nesting materials and you won't get chicks.
My Alexandrines live very happily together. They squabble from time to time, but are generally the best of friends. Each is firmly bonded to me and each has its own separate personality as well.
There is *always* the possibility that a pair of birds won't get on and will fight. It's up to you to minimise that likelihood. Having said that, though, every animal behaviourist on the planet knows that relationships always begin with what we in Oz call 'a bit of a donny'. That would be a bit of side-eye, a bit of a whack, a bit of 'ow's yer farver and eventual harmony.
You wouldn't just plop a second bird into the established cage of a first bird. You'd need to plan the introduction and remove Bird #1 from the shared cage for a few weeks beforehand (that way, Bird #1 is less likely to view the shared cage as 'mine!'). You'd introduce the pair together into the shared cage, making sure there is plenty of room for both birds to 'own' territory (such as perches, water, food dishes etc). If the cage is small and toys not plentiful, then there will naturally be arguments over stuff.
Y'know, before people began anthropomorphising birds, aviarists just plopped them (birds, not people) together into cages and they (the birds) worked it out. Same thing happens in nature. It's like raising kids: you simply can't (and shouldn't) protect them from getting hurt in everyday life. They need to develop resilience and discover ways of negotiating. (NB. Of course, you're not about to stand by and watch murder occur either, but in my experience it rarely devolves to that).
In the end, the decision is yours. There is always the possibility that you'll end up with another cage to clean and two oppositional birds, but if you're careful and plan ahead that likelihood should be small.