Application.open

Document open (File document, any as, bool asSmartObject)

Parameters

document	The document(s) to opend.
as	The document type (The default is to let Photoshop deduce the format).
asSmartObject	Creates a smart object around the document.

Description

Opens the specified document file(s).

var doc_a = app.open(new File('path/to/file_a.psd'));
var doc_b = app.open(new File('path/to/file_b.psd'));

if (app.activeDocument === doc_b) {
    app.activeDocument = doc_a;
}