I received an email about this with some more details, so I'll post an answer here.
As I understand, you are trying to randomly select pairs trials. The first member of the pair is randomly selected, and immediately after it is run, the second member of the pair is then run. After the second member is run, Inquisit randomly selects from the first members of the pairs again, and so forth.
The way to set this up is as follows. The /trials command of your block should randomly select from the first members of all of the available pairs:
<block practice>
/ trials = [1-8=noreplace(a1, b1, c1, d1, e1, f1, g1, h1)]
</block>
Each of those trials should have a branch that runs it's corresponding second member:
<trial a1>
/ branch = [trial.a2]
</trial>
<trial b1>
/ branch = [trial.b2]
</trial>
<trial c1>
/ branch = [trial.c2]
</trial>
and so on.
Pretty simple. Hope this helps,
Sean