You don't know that though (unless you can see into the future or have a camera hidden in the dev's room
).
Sure, it's been shown that SnowStorm continues off from each girl's "Normal Endings" but that doesn't necessarily contradict what GleenX himself stated in-game (at the end of Maddie & Nora's "Special Endings") with those endings somehow having some degree of influence on SnowStorm.
I mean yes, it could very much be addressed in a spinoff game, or it could be addressed in his third EraStorm game (which along with SnowStorm still counts as "future episode"), but unless GleenX himself specifically says "Nope. They won't be addressed in SnowStorm", you'll forgive me that I don't take your word for it (I'm not saying it will definitely happen, but not dismissing such possibility either, keeping an open mind).
Given that portal has massive amount of magic with it (as well as gods like Anubis and Sekhmet appearing in SandStorm), it's not really that much of a stretch to think magic transported SandStorm's MC from modern day Egypt to Scandinavia of the Vikings age.
Sure perhaps it could go down as you said (and SandStorm MC and SnowStorm MC are completely separate characters), but given MC in SnowStorm has amnesia, and is found in the exact same state (collapsed in snow, unconscious) as SandStorm's MC ended up (collapsed in snow, unconscious), I just find it too much of a coincidence the two MC ended up and start in their respective games. So it's hard to imagine they're separate characters.
Of course, there's also the eventual situation that SnowStorm MC will regain all his memories.
In my eyes, it makes for a much stronger narrative for him to be same as SandStorm MC, because most of the players for this game (unless the player skipped SandStorm and jumped on to SnowStorm) saw the relationship that character went through for that first game.
Just a quick flashback scenes from SandStorm of him in SnowStorm remembering his past, without that much backstory needing to be told.
Whereas if the character was separate from SandStorm MC, then I'd imagine a lot of backstory will need to be addressed, and players will probably not feel as connected to possible LI that this "new" character may have had before (that is, if he had one).
