Don't get me wrong with RT. I love RT - it is a great line with a different feeling than DH.
BUT. Rules-wise, it has a big overlap (if you subtract the voidship rules), and it only thins out the existing lines if you create more and more. And you avoid redundancy (or worse, inconsistancies)
If you add just a handful of things, you could easily merge it:
> Voidship components and requisition of these ships/parts (take existing lists from RT and add some Askellon flavour to it)
> Voiship combat (could be taken nearly 1:1 out of RT, if too lazy to rework it )
> A Commander role
> A Navigator role or Elite class (IF you keep Navigators at all...to be honest I never really understood why such precious assets risk their lives outside of a ship EVER; and Navigator-only-action tends to bore the other players)
> a handful sanctioned Xenos classes for those who like it (esp. Kroot Mercenary, Ork Freebooter, and maybe a Dark Eldar, Eldar Ranger and/or a Tau)
This would easily allow to play RT with the DH rules, and it would even provide cool stuff from DH for your RT campaign.
And it very well should be possible to put it into a book of around 150 pages, if you keep the fluff at bay.
Lets sign a petition ;D