I am having a similar issue with an experiment I want to run with directRT 2010.

I am trying to randomize a task in which there are 24 trials with sets of images within each trial. There are images of 4 people, each with 6 different types of images (which make 24 trials in total). I want to make it so that all 4 of the same type of image do not occur all within the first half or all within the second half of the task. I do not want to constrain it so much by making 2 blocks and putting 2 of the people within the same block, and then just randomizing the order of the blocks (because there may be some effect of always putting the same 2 people together). Do you have any suggestions as to how I could do this?