Dear YM,
When specifying single-subject models, go with two separate sessions. You can then also specify the corresponding volumes, conditions, onsets, and so on. I haven't tried myself, but it should be possible to also load one 4D file for the first session and another 4D file for the second instead of loading two series of 3D images.
BTW, make sure that the two sessions have the same scanning parameters and that they have been preprocessed the same way, otherwise the differences might be difficult to explain.
Best regards
Helmut
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