I have several reasons for the way that I answered this question, so I will try to explain a bit better.
The very first thing that I feel that it is important to consider when choosing birds when it is your desire that they become friends enough to be let out together is the size of both the bird and the size of the birds beaks. While many times it is true that the smaller bird is the faster of the two when there is a difference in their sizes, so it can often move quickly enough to escape from the larger bird as long as it is aware that it is in danger. It is also true that the smaller bird stands a much greater chance of being severely injured or killed by the larger bird. For this reason it is a normal practice to advise people to keep the bird to a similar size or to not let them out together, it is what I actually recommend, as the majority of people find it difficult to keep as strict of supervision on their birds to make it safe enough over a long period of time, humans tend to get lax in this area especially when there have been no incidents after a period of time and that is when the majority of injuries or deaths will happen, and we all tend to believe that this will not happen with us which is also a mistake. with these birds it only takes a momentary lapse in our vigilance for this to happen.
We must take the time to consider not only the sizes of our birds but also the basic natural behaviors of the different species of parrots as it is in their natural environment as that can be one of our biggest clues as to whether the birds can or will tolerate each other. In this case both species live in a totally different part of the world so they would never even be aware of the others existence. Both of these species of parrots are aggressive and do not usually have anything to do with any other species of parrots in their natural habitat. Both of these species are very protective of their mates and their nesting areas and territories, Like the differences in their sizes, these traits do not suggest that bringing these two species into close contact is going to work well for either of them.
In the negative column I have one last thing that should be considered concerning these two species and that is that if either of these two species has been accustomed to being a lone bird for any length of time, they can be very difficult to find a suitable mate for as they will very often reject the other bird even when it should be an appropriate mate for them and that also tends to lead to the injury to both birds and usually the death of one of them.
Still, I feel that I must be honest with you by saying that it can be done, although it is not recommended. I know that this is true simply because I have these birds myself, but I really don't recommend this combination for any one. honestly, after giving this more consideration, I probably should have just kept my mouth shut or just said that it is not recommended rather than saying that it can be done, All it takes is one little slip and the results could be disasterous for the parrotlet.