If you are playing on a US account you will need to buy a season pass using that account. For Shadowkeep itself you could buy it on the other account and still game share it but you wouldn’t be able to game share the free season pass that it comes with and all future SP’s would need to be bought by the account you play on. Personally my recommendation would be to get as much as you can on the account you actually play the game on. Come October 1st your play account will be able to acquire the year 1 content for free. If you don’t need to play Forsaken content then you could just buy Shadowkeep as it doesn’t require Forsaken to play.