Consider for example the double parshah Vayakhel-Pekudei. The aliyos from the two individual parshiyos are combined as follows: 2, 3, 1, 2, 1, 2, 3. (Meaning: the first two aliyos are combined into one, then the next three are combined into the next aliyah, etc.)

Why not just combine each pair of aliyos from the separate parshiyos into one? (As far as I know, the only pair in which this is done consistently is Acharei Mos-Kedoshim.)<!---->