The wording around expiration dates is so poorly written that nobody has been able to figure out what they were trying to say. If you get one referral it seems clear that they intend to give you 5,000 miles which must be used or forfeited within six months.
But what happens if you get a second referral three months later? Who knows. They didn’t bother to write it in a way that anyone could understand so it’s anybody’s guess how that will work. It could mean that all 10,000 miles get extended six more months. It could mean that the second set of 5,000 miles get six months. Or it could mean you don’t even get 10,000 miles but rather you just get the same 5,000 miles total but get six more months to consume them.