Usually there would be another variable to indicate which group a value is in, but it all depends on how you designed your survey. If the values are all pre/post/pre/post you can compute the group variable according to whether the case number (a special variable named $CASENUM) is odd or even. Use the mod function with modulus 2 to do this. (Start with the Transform > Compute menu).
If all the pre values are followed by all the post values and, let's say, there are 100 of them, you could compute the group number like
compute group = $CASENUM > 50.
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