i'm pretty sure it was like that in 6th edition too (though i may be mixing that up with fantasy...) but i find that everyone i play with rolls them all at the same time, probably just to save time setting up
just so you know, even if you do swap your first roll for the primaris power you can still roll further on that table, you just cannot swap for the primaris (though if you do roll all your dice on one table you can't swap for a primaris power anyway, unless you're chaos) edit: actually i'm not quite sure about this but the relevant rules are quoted belowThat 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.
and i think that, from the way the rule book is worded specifically the "if the psyker has chosen all his powers from the same discipline", you have to pick what disciplines you'll roll from before you roll any dice:Remember though, that the Psyker cannot have the same power twice - if he chooses to substitute his first power for the primaris power, and then chooses to generate another psychic power from the same discipline, he cannot further substitute powers from that discipline
if a psyker has chosen all of his powers from the same psychic discipline to gain Psychic Focus, he will already know that disciplines primaris power and so cannot substitute any of his randomly generated powers