Welcome to the forum.
Are these two separate people whose service records you are looking for, or just one? If you are talking about two different people, roughly when would the first person who you mention have served? (Don't post any distinguishing info e.g. name etc if that person is still alive, though.)
There is info on here about how to apply for relatively recent service records:
https://www.gov.uk/get-copy-military-records-of-service