Once the new/friend/referred account has paid for 2 months subscription, log on account management on the old/referring account. Under the refer-a-friend tab of account management, you'll be able to see the status of the referring account and what gifts (if any) you've earned. If the mount icon is there, you can click on it to choose which server, then which character on that server, will get the mount. After you confirm your selection, the mount will instantly appear in that character's mailbox. It, and the box it comes in, is BOP so it cannot be transferred in any way.

If you've just purchased the accounts, the only way to get the mount 'early' is to use prepaid game cards. That subscription time will be attributed to your account (and towards the referral) immediately, whereas if you use the standard credit card subscriptions, the months purchased will not be credited until they've actually been charged. You'd have to wait the initial 30 days to be over, then purchase a 3 month subscription to get credit or wait 60 days from activation (30 original days, + 1 month recurring subscription and its first renewal).