It's simple: it doesn't matter where you purchase the subscription from. You will be able to use both services with any subscription. Your fast GPU hours are used up by both services. So if you are on the Standard plan are you use half of your fast hours with Niji Journey (7.5 hrs out of 15 hrs), you will only have half left (7.5 hrs) to use in either Midjourney and Niji Journey.