Maybe this is changing the subject but where do you get that twins were common in Archenland? It's true that there were naming conventions for twins (Dar and Darrin, Cole and Colin, Cor and Corin) but there were also naming conventions for twins in world at some point. (Clement and Clementine, Benjam and Benjamina, etc.) Actually, the Archenland naming conventions Shasta mentions aren't even for twins specifically, just brothers. So I don't think we can assume twins were more common in Archenland than in other countries. 

Posted by: @jasmine_tarkheena

Calormen was known to be ruled by "The Tisroc", a title given to the King of Calormen. Though we can't be certain of who was the first Tisroc. While in The Horse And His Boy, Aravis mentions that one of her ancestors, a Tisroc, was in line from Tash, I think she was exaggerating or it may have been a way of bragging.

I assume that what Aravis mentions there, is part of her family history (or perhaps family lore) - something she's been told, perhaps good for bragging, at least for some pride. She might not be exaggerating about what she has heard, but there's always the possibility that the details of her family history may not be completely historically accurate.

We don't know how many generations back this ancestor lived - the accuracy may suffer if there are many centuries in between.

Posted by: @jasmine_tarkheena

What do you think might have happened in the history of Archenland and Calormen?

I'm not entirely sure, but i read somewhere that Calormen and Archenland were founded by Frank's sons. Or well, Archenland. I think i remember that Calormen was founded by bandits from beyond (possibly Telmarines) Which leads me to think that Telmar is much older than Calormen and that would explain the evil tendency of the Calormenes.

@giigipevensie 

According to the Narnia Timeline (which may not as well be entirely canon), there were outlaws from Archenland that established Calormen. Sometime later, some Calormenes went to Telmar. However, Aslan turned the Calormenes in Telmar into dumb beasts for their wicked behavior.

So it might be safe to say that outlaws from Archenland made their way across the desert and established Calormen. Also, I would say that when the White Witch took over Narnia, the humans could have fled to Archenland. During the Great Winter, Archenland and Calormen were not affected by it.

@giigipevensie In The Magician's Nephew, we're told near the end that Frank and Helen's second son became king of Archenland. However, in the Narnia timeline that Lewis apparently drew up some time after finishing writing all the books (it wasn't published until after his death), we're told that Archenland was founded in the Narnian year 180 by Prince Col, younger son of Frank V of Narnia. The timeline also gives info about the founding of Calormen and Telmar, none of which is in the books at all. But because of the discrepancies between the timeline and what the books themselves say — in this instance and a few others — there's no final answer on what should be considered "canon".

The unfortunate fact is that Lewis wasn't very careful with his world-building and didn't plan out very much of it in advance (and apparently didn't always check back to confirm what he'd written earlier), so even within the books themselves there are several significant contradictions between the earlier and later books in the series. We do know that Lewis was aware of at least some of those issues, as when the editor of Puffin Books was planning to bring out the first paperback editions of the Chronicles, Lewis stated that he'd like to go over them and change some things that didn't match up. But unfortunately, that was literally only a couple of days before he died, so he never got the chance to do it and we'll never know what edits he might have made!!