Hi guys, I'm looking for a book for my friend who's been searching for it for ages, and I'd really appreciate it if you could help!
She doesn't know the title but what she remembers is as follows:

The dragons have no wings, and they fly backwards when they release a jet of fire through its mouth

There is a boy and a girl sitting on the dragon on the cover

The dragons' names have no vowels

There is a portal which links them from the dragons' world to the human world

In the dragons' world, there are two clans which refer to each other as Barbarians

If you can remember the book, please tell me, your help will be very much appreciated! Thank you!