They are just rectangles so group each card so none of the parts move, then treat them as simple boxes.
Draw crop marks outside the corners (but not touching) using the pen tool. Then group the postcard with its crop marks
Best is to accurately centre the designs in the middle of the sheet, with whatever gap between you need to trim them. In the USA that would be 0.25 inches anywhere else 5 or 6mm.
Format > Arrange > Position
Depending how you are going to print them and if they are front and back designs, you can set them up with two front, two back as Print and Turn (the paper is flipped horizontally for the second side), or Print and Tumble (The paper is turned over vertically for the second side).
Check a copy, taking notes of the steps you take printing and the results you get, fixing whatever needs doing. You may need to rotate the backs 180° in relation to the fronts to get them to come out the right way after printing.
If you are doing this on a home printer and not using a Commercial printer be aware that desktop printers do not position the paper accurately and the paper can be quite out of place, so do not expect accuracy.
After printing cut along the crop marks.
Peter