If the Tyrell’s managed to get Sansa out of KL before she got married off to Tyrion – and was married off to Wilas in Highgarden – would their son have been lord paramount of both the North and The Reach? How come two great houses had never had an heir like thst in 300 years?

1. Yes, although more likely they would have split the titles between first and second sons. 

2. Because there’s a cultural taboo against combining land holdings – “Holdings are seldom divided. Nor are they combined, as a rule, although one person could concievably hold more than one title.” – which makes sense from a geopolitical perspective. It’s hard to administer two far-flung holdings, and there would be a good deal of tension about favoritism, where the heir will be raised so they’re not considered a “foreigner,” etc.