Without much in-depth ID knowledge, a brute & tedious approach could be to export all individual items individually and save with generic incremental file-names, keeping front & back as separate files ( if indeed necessary ) and use an imposed template with yet another variable data text file which references the previously exported files. With some strategic pasting in Excel, I believe this could achieve your desired result, but I would like to see what true Indesign users have to say about your question.
My XMPIE plugin that we had did everything Im asking. So I guess Im just confused why I would design a card in InDesign, export it as a jpg out of InDesign, and re-import it in to InDesign for every single card. It just seems odd to do. It will also increase rendering time I would think to do this. The mailer itself is the same for all people receiving the mailing. The only thing that changes is the mailing information.
The way I described is a "dumb" way to put multiple records on a page, so it would definitely take more time all around, and it's only a step above exporting all of the items and then manually imposing each one. It just takes care of the imposition by leveraging the variable spreadsheet to place your files inside imposed graphic frames.
First off, XMPIE is far more efficient at printing than Data Merge because it processes the document once at the printer and merges the variables on the fly. You can send a single page file and get thousands of prints.
Data Merge requires that every record appear on a page in the file you send to print, so you are processing the static data over and over for each page.
That said, how is your data file set up now? A single set of data in each record, or data for 4 people in each record for printing 4-up?
What problems, exactly, are you having getting the multiple up to print?