Howdy y'all
Heres a round up of everything I can find about the mirrored pair cards in Reality Fracture
The product page, https://magic.wizards.com/en/products/reality-fracture, shows the booster packaging which says "2+ legendary cards in every pack".
Heres Maro confirming that every pack will have a mirrored pair: https://markrosewater.tumblr.com/post/815565310132453376/is-there-going-to-be-a-mirrored-pair-in-every
From this we can deduce that all the mirrored pairs are legendary cards. Which eliminates [[Snapcaster Mage]] and [[Stingcaster Mage]] from being a mirrored pair. EDIT: Heres Maro saying that: https://markrosewater.tumblr.com/post/815472247785570304/you-said-that-each-pack-of-reality-fracture
Despite the one mirrored pair weve seen containing a reprint, most will not have reprints: https://markrosewater.tumblr.com/post/815595888760225792/do-the-mirrored-pairs-always-contain-a-card-that
Heres Mark being evasive about if each mirrored pair will have the same card type (ie creature and creature, planeswalker and planeswalker): https://markrosewater.tumblr.com/post/815509587743686656/can-i-get-a-hint-if-any-of-the-mirrored-pairs-wont
According to this thread, https://www.reddit.com/r/MTGRumors/comments/1sq05ml/gamegenics_product_listings_for_reality_fracture/, Gamegenic has product listings for albums and playmats and stuff feature planeswalker 1, 2, ...., up to 11.
This revealed art for "Face Yourself" features 11 figures; Jace + 5 mirrored pairs. I think its safe to assume that these are planeswalkers 1-11. And from there I think its reasonable to assume these are the only planeswalkers in the set. Assuming Jace is colorless/5 color and they have balanced the colors, Vraska needs to be green and alt Vraska needs to be blue.
The two revealed Chandra's we've seen, [[Chandra, Chill of Compliance]] and [[Chandra, Torch of Defiance]], have collector numbers 212 and 244 respectively. These numbers are too high to be in the normal position of a normal sized set. I suspect that all the mirrored pairs will be grouped together at the end of the set, either immediately before or immediately after basic lands.
The Chandras are 32 cards apart and 2 colors apart. If we assume they are in the same part of their color (does red or blue have dramatically more characters that come before Ch- in the alphabet?) and that the colors are evenly balanced this would mean that each color has 16 cards. Assuming all mono-color cards in the paired section that means 80 total cards in the paired sections, or 40 mirrored pairs.
If chandra is the start of both blue and red that would mean the final green card from the pair section is 275, which is about the number of cards in a large set (SOS had 276, ECL had 273, TLA had 286, EOE had 277)
The magic wiki lists 24 known characters who have been desparked, https://mtg.wiki/page/Desparkening Weirdly including Liliana Vess. Its possible that the gamegenic packaging refering to numbered planeswalkers is simply calling her a planeswalker for convenience reasons and she'll actually be a creature, and who know about her (or anyone elses) mirror versions. Also maybe Liliana sparked up again :\