How to combine two 1-page documents into two-sided print?

I have two 1-page documents and double-sided printer. I can print them separately but prefer to print them on two sides of one sheet. The documents have different formats and cannot be merged by their editor tools. Is there a technique to append one print page to another at printer level?

There are PDF, XPS and print to file options which seems attracitve candidates for page shuffling.

Edit 12 Nov 2015: I considered the mechanical alternative, where you print first surface, return the paper into the tray and print the second. But, I would like to avoid it because you need experimentation: you never know which side is printed first. This is aggravated by enterprise printers, which are located far away and used by other people as well. You can spoil their work if place you half-printed sheets at wrong side. I therefore ask for electronical merge/queuing.

One way is to print into PDF and use Adobe Acrobat to combine files, one after another

Hopefully, there are more open SW solutions.

You're asking about duplex printing. You just need a PDF with two pages in succession.

PDFtk is open source. You can download a portable version of PDFtk where you can use it to merge PDFs into one document. This will allow you to perform the duplex printing. I think your best results will be when all documented you want to duplex are of the same dimensions.

You may get odd results with different dimensions (one side tabloid 11x17 and another side letter 8.5x11) depending on the PDF reader your use to print and how to printer decides to handle different page dimensions.

For me, the best print settings for duplex of odd dimensions ended up being this in Acrobat XI: