myDoc = app.activeDocument;
mySpread = myDoc.spreads.add();
Jumpy, why are you removing a page in the last line?
Is .spreads.add() equal to adding a new page also?
> four 8.5x11 pages set up as spreads on a single pasteboard
Is that a single spread with four pages, to be split in two spreads with each two pages?
(... after trying ...) Oh I think I get it.
You need to move pages to another spread after the only one, and to do so you need to add a spread first. You can move pages around but not "to" a new spread. If you do so, you only move the pages around inside the current spread.
But you can move two-out-of-four pages in a spread to a 'new' spread in the Pages panel ... just not with scripting ...