As I feared, there is a reason if nopony managed to identify those two characters from just the first half of season 5.
When the poster was released, all the community assumed that since some of the character were obscured/blackened out that would mean that the visible characters where ponies that already appeared in season 5 (episodes 1-13), while the other ones would appear in the future episodes (episodes 14-26). While this assumption is mostly true, it doesn't take in account one thing: once again, Hasbro marketing seem to have done one error.
The two UMP (Unidentified Mare Pony) are in fact two characters that have appeared in episode 15 "Rarity Investigates": no wonder no one identified them back then!
The pegasi on the left is no other than Spitfire mother, Stormy Flare.
The one on the left is Cinnamon Chai, the cake shop owner from the same episode.
These screencaps show them as they appear in that episode.
