Ooh, rare to see Kalafina music on here. Good taste, and very welcome. I've been listening to Kalafina and Kajiura's music for a good many years and covered some myself back in the day :)
Onto the review! The performance is the piano is beautiful. It's gently and emotionally played. Kanade's voice is also very nice. A lovely tone and ear for harmonies which obviously lends itself well to a Kalafina song. This song would benefit from better mixing I think. While your piano is an accompaniment to a singer, it could be mixed in a way that allows the piano and the vocals to shine equally. I think the piano could afford to be a bit louder so that it really packs a punch. Your beautiful performance is often covered by the thick layer of voice.
Which, moving on to the vocals, they're really in need of some reverb! Kalafina songs always have this lush, almost cathedral-like reverb that makes the harmonies sound so powerfully rich and gorgeous. The reverb softens the vocals a lot and will give this an otherwordly sound. I also think the piano could benefit from a thicker reverb too. I think there's a lot of space in the mix.
I really hope you don't mind, but I've downloaded the song and just mastered it a little, put a bit of reverb on, just to hear how it would sound if the mix wasn't so dry. Ideally I wouldn't just throw a reverb onto a whole track, as it can get a bit muddy, but I think it does a lot for the track, personally. She has a very Kalafina-like voice that just suits itself really nicely to a thick reverb.
You can listen to that here: https://www.mediafire.com/file/fqu9vcyp15u73zu/Allelujia%20with%20Reverb%20-%20Jabicho%20+%20Kanade%20Lacus.mp3
I hope my review doesn't come across negatively. This is one of those few times that I feel that my own musical background and experience in working on a lot of this type of music can actually be of some benefit. I had a listen to some of your other collaborations too! THOSE STRINGS. You guys seem to be having a lot of fun and you guys collaborate beautifully. You've both earned a new fan :)