As far as I know, you can do a single-record-per-page merge on multipage documents and put the fields anywhere you like. ID should make duplicates of all the pages for each record, though, so this is a very in-efficient method of making a document where you have one page of static information, and one page with variable info.
Data merge will not work, I think, if you have a file set up with 500 pages and expect to merge onto 250 of them, though. That just isn't how Data Merge works.
Is it possible to create a data merged document with the variable data only on every second page?
I have set up the pages with the variable data as a master page (on every odd page) with the text box and paragraph style all set up, and I have set 'override master page items' so the data can be placed, but InDesign seems to freeze up when I try to create the merged document?
Perhaps you can modify the data file so that empty data items appear between populated data items, and add a data field to the output document.
Or, perhaps you can redesign the document layout to be a single page that's the size of a two-page spread.
There should be no problem doing the merge with a two-page master and a two-page template document with fields only on one page (and off the top of my head there's no reason to move them off the master page if that's where you put them). I think the problem here is that rachrachm already has a file of 500 pages (based on another thread asking how to apply masters to every other page).
It's potentially possible to simply place the data file (without the header row) as a text file and auto-flow through the master page frames (can't say for sure without seeing the files), or if the pages without the merge fields are the same just, remove all but the first two pages from the template and do the merge.
As I metioned earlier, though, this is a very inefficient method of producing that sort of document. A true variable data print flow would be better (you'd need to find a commercial printer who can do it with your data file), or the home version would be to make one single page file for the static content, print 250 copies (plus a few extras, just in case), then make a single page merge template, do the merge, and print that on the back of the already printed pages, or as a separate page. It is immeasurably faster to print multiple copies of a document than it is to print a document that has multiple identical pages.