Just buy it for the one platform you want to play on. If you don't buy it for the other platform, you won't be able to log in on that platform if you log out of your main platform in an Endwalker area; however, if you're only playing on the one platform, this would be a non-issue.

Quote Originally Posted by EveK View Post
Might even need to wait till the official launch on the 23rd.
This has never been the case in the past. Anyone who pre-ordered Expansion-Only received all expansions access at Early Access Friday. Only Complete-Edition people had to wait until Official Launch Tuesday because they strictly speaking didn't pre-order the expansion.