See at Colin Flashman's blog. He has a lot trickeries for datamerge including facing pages merging :
That is kind of how i have done it now.
i creted a new Document without facing pages and recreated everything from scratch.
It wasn't that easy since every page had an image and also Text across both pages that also had Datamerge tags inside of the Text.
So i've learned for the future... no facing pages for Datamerge !
I was hoping that i could avoid recreating the whole document / splitting text and images so they will fit 100% etc.
The Job is done now - and iam still not sure if this is a feature or a bug.
I don't think it's a bug. IMHO it's just people expecting more that datamerge can provide. People forget datamerge is mostly intended for business cards or similar projects. It's great if more can be done but definitively not what was planned.