That is correct, one at a time. 7th did bring one oddity though related to psychic focus.
It is illegal to of picked the primaris and to have psychic focus. So you get the following situation:
A ML2 psyker rolls scrier's gaze on the first roll and swaps it for the primaris (prescience). The psyker now CANNOT roll again on divination as this would give him psychic focus and as a result, cause an illegal state because the primaris was swapped for. Which is explicitly disallowed.
So, rolling one at a time is even more important than before. The impact is, if you are going for a specific power, do NOT swap for the primaris, even if you plan on bailing from the tree after you roll it. By swapping for the primaris you remove your last role as a chance to get the power you want. Which, we all know is invisibility
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It is illegal to of picked the primaris and to have psychic focus. So you get the following situation:
A ML2 psyker rolls scrier's gaze on the first roll and swaps it for the primaris (prescience). The psyker now CANNOT roll again on divination as this would give him psychic focus and as a result, cause an illegal state because the primaris was swapped for. Which is explicitly disallowed.
So, rolling one at a time is even more important than before. The impact is, if you are going for a specific power, do NOT swap for the primaris, even if you plan on bailing from the tree after you roll it. By swapping for the primaris you remove your last role as a chance to get the power you want. Which, we all know is invisibility