randomising location within pair (case 29182)

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Hi

I am presenting two items on the screen at the same time. One on the left and one on the right of the screen. Each pair will be shown 6 times, once in each of 6 blocks. For each block I want the presentation of the pair, which one appears on the right or left, to be randomised but so overall across the 6 blocks each item is presented three times on the left and three times on the right. I have scoured the manual and have no clue. Any help would be appreciated!

Thanks