iPad as input device for iPad

I got a retina mini for christmas, and I already have a 2012 mini.

Picking up the perennial thread of a dual screen laptop, are there any apps that let a pair of iPads be used in tandem? It seems like a natural fit for a number of use-cases where a totally fluid (e.g. keyboard one moment, colour palette the next, formatting toolbox after that) input mechanism would be especially useful if it didn't obscure half the app's content area while it was up.

You know, some kind of unholy union of the dual-screen Libretto and the ill-fated Courier, made from a pair of iPads.