can i still get the flying mount

To answer your questions, yes, you will still get the mount when that account pays for a second month.

This is true even if the in-game benefits have since expired.

***Quoted for Turtle***

As Zouche pointed out with the linked Support Article, you are able to gain access to the X-53 Touring Rocket even after the 90 day link has expired. This is true for all account based rewards - in essence the 30 day game time reward for any player that you recruit who upgrades to a full retail version of World of Warcraft and the rocket mount