Hiya! I'm seriously still fresh as a newly baked cake with parrots but so I'm FAR from an expert but I'll answer in a super basic way, based only on what I've been reading around these forums & research!
Parrots are, as you undoubtedly know, flock animals. This doesn't necessarily mean however, that every type of parrot who has a mate will have a blissful relationship. Poicephalus parrots, at least from the research I did, may at times, after years of bonded mate bliss suddenly decide it just isn't working out and MAY develop some... shall we say, "martial problems" (just keeping the wording light hearted to emphasize the fact that I'm no expert here!)
HOWEVER: Here's an article I found about the mating relationships of parrots:
http://pets.thenest.com/parrots-pair-life-12861.htmlPLEASE do forgive me if this article is somehow incorrect or misleading! I just bookmarked it and decided to fish it out since it had this to say about Poicephs:
"Most parrots pair for life. They form strong bonds; once they do, they should not be separated. Scarlet macaws, lovebirds and Puerto Rican parrots are examples of monogamous parrots. Poicephalus parrots, however, can be fickle. BirdChannel.com reports that these parrots in the wild are known to switch mates after years together. Poicephalus kept as pets may grow aggressive toward each other after years of bliss. It can get bloody when Poicephalus parrots decide their relationship isn't working out."Quoted directly from the article.
Here's another interesting link I found pertaining to the behaviour & mating of jardines:
https://www.beautyofbirds.com/poicephalusparrotbreeding.htmlBUT! I'm not an expert on these guys; and I hope that the links I managed to provide will at least shed some light and bits and bobs of info? If not, I -PROFUSELY- apologize. I have a huge list of these links saved up just in case. SO ALL YOU LOVELY EXPERTS WITH YOUR LOVELY FLOCKS! Shed some light here

Let us know which links give actual accurate information! Share us your experiences! People on these forums have recommended that I get a lady friend for my lovely green baby, which is what I intend to do. (though oddly enough I've even gotten suggestions that he could be just as happy with a male buddy, or even one of a different species but... I myself am leaning towards a lady ringneck.
And as a purely newbish question: I'd love for some jardine experts share their opinion (XD and bash the links if they give bad info!) But yeah, all I can recommend is RESEARCH! Read up on it. BUT MOST OF ALL! Ask these lovely forum people. They have been absolutely welcoming and very kind and straight forward about any questions I've EVER had. They all get answered, and I get lovely advice too. Research
(I'd love to learn more too because I want to know more about different parrots I myself!) After all, not all info out there is accurate. Far from it - different owners and people who have more, actual experience with these guys may shed some better light on your lovely jardine

What I don't know is how they'd behave with parrots of other species at all; they MAY or MAY NOT need a "mate" that's of the space species? I
-HONESTLY have zero clue about this.
FOR NOW: I'll let the lovey experts weigh in here

! AND GOOD LUCK finding a new lovely flock candicate! <3