I have a sennie and a quaker, and I have to constantly watch over them when they are both out at the same time. We had an incident a year or so ago where my sennie pierced a hole in the quaker's beak. Luckily, after a vet examination and some cream for the beak, the hole did not cause any problems and eventually grew out.
I don't have either but if you did have them out at the same time, as with any pair of birds, I would just monitor them. I would also try to get them used to each other in neutral areas (ie not near their cages and in unfamiliar places) so that they don't feel defensive or territorial.
I have a cockatiel, Nanday Conure (twice the size of a GCC) and a Senegal that are out together. I keep a camera on them and never let my guard down. My Senegal thinks she is huge and will never back down from a fight with the Conure, who can be grumpy. It is constant monitoring every few minutes. They won't fight if they know I'm watching! Naughty birdies.
I keep parrotlets and a plumhead. (similar size difference between your two..though both smaller parrots). Parrotlets are also too brave for their own good. I allow common flight and play with the whole flock from time to time, but it does require 100% supervision. Generally in a large enough space birds can fly away from things they don't want to be involved with. Multiple perching areas help. There is no guarantee that this will always be the case. When an accident does happen it can be fatal to the smaller birds. The most injuries I have ever really had have been via the cage. Parrotlet 1 lands on parrotlet 2's home and they bite each other..fiercely. Birds get injured very easily through cage bars because their toes and ankles are directly inline with the other bird's beak. Something that isn't as easy when they are perched and lunging at each other. There have been people in this forum with similarly sized birds and a bird lost their foot from being bit through cage cars.
My two parrotlets have squabbled during common play time, there have been feathers flying and screams. Though they are the same species and the damage was minimal. If a similar squabbled happened with a larger bird the parrotlet would have been killed.
Common play is possible, but there are many factors to consider. The bird's personality, the season (breeding season can be more emotional for all birdies), the space available, the amount of perching areas for birds to get some alone time if they want it. You must accept that their will be risk. You can do what you can to minimize it, but don't let your guard down. You will also need to make independent out time for each bird in the event that they can't get along. With my flock all birds get their own time out, common fly time is more of an event and my partner and I are both home to watch them. When they are flying together it is beautiful, they flock together and fly from one side of the house to the other. They are very motivated to fly with my plumhead and he LOVES flying so the whole flock gets an exercise routine.
My Senegal would even bully my amazon to the point of the amazon beeing utterly afraid and bleeding! The Senegal would just sit and watch her handywork very proudly.. oh my! I would never trust a Senegal with any other bird except for maybe a mate hehehehe