I've never seen an interspecies pair having sex but they do act in every other way as if they were mate bonded (male feeds female in the beak, protects the nest, they are always together, they allo-preen, etc) so, although I don't know whether this type of relationship is 100% satisfying to them (because I don't know if they actually do have sex), they do present the normal (natural) behaviors of mated birds in the wild. And they do love each other (you should see my peach front male with my female GCC, they sleep with their sides leaning against each other, the cutest, cutest thing!). And, yes, they would have to share a cage - after they are bonded, that is, not before. Keeping them separate would be cruel and they would let you know they are not happy (Pablo, the peach front male, screams when Codee, female GCC, leaves the room on my shoulder and runs to her as soon as she gets down from me and climbs up a cage).
And it is as Wolf said, first you need to quarantine and get the new bird seen by the AV (your jardine has had a physical, right?) and, once you see she is fine, you would start by putting the cages near but not touching and, then, if you see they like one another, you can put them side by side and see what happens because it's real easy to tell if they like one another or not. Now, a word of caution, just because they don't like each other at the beginning, it doesn't mean they won't in a few months. Sometimes, they are jealous of the attention you pay the other bird when the new bird arrives but, as they get used to one another, they start liking each other.





