If you open the full page editor, the greeting card controls are on the right in the size/loc tab.  I assume you only need to center within the top half and bottom half based on your screenshot so you only need to select the top print and then click the top/middle button in the circled controls and then select the bottom print and click the bottom/middle button.

Regards,
Mike

I guess I'm not understanding how this is a problem.  You have a special case of centering where you don't just want to center an arbitrary print size within each half but you want to maximize the size of the print in each half minus an 8mm border.  There's no denying you'll need to do a little math one time to figure out how big to make the print so that it leaves 8mm around the print on each half.

But once you do that, you save a layout and never have to worry about it again: it's 2 clicks to use the layout in the future.  Or even if you don't save it as a layout that you can bring up in one click any time, once you figure out that you need a size of 193.9 x 132.5mm, you could right click in the print size list and add that size if you use it often.  Then you can choose that size any time you like on an A4 sheet and use the two previously mentioned half-centering buttons to center it.

There are even other ways to easily do this with no math but I would think creating a layout you can use any time per the above it best.  But you could, for example, switch to borderless in the driver and then right next to borderless in the driver, select "Settings" and "Retain size" which will turn off expansion.  Now you can just set your print size to half of one A4 sheet (210x148.5), use IntelliCenter placement, and use an 8mm inner border.  This method has the benefit that you could specify any border you like at print time: you could go up to 10mm for example.  So doing it this way, you could right click in the print size list, add a 210x148.5mm print size, label it "half A4" or even "A5" if you like, then any time you want to do a folded card on A4 paper, use that size and specify the border you want by just entering the border (like 8mm) in the inner border setting.  You don't even need to use those half centering buttons in this example because IntelliCenter will do that for you.  The disclaimer is that you should do this with the prescribed borderless settings in the driver (save a printer setup).

But again, if you don't have reason to do arbitrary border sizes at print time, the easiest thing is to create a layout and just load it when you need it.

Mike