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Ok, so I just recently found out that you can in fact roll on different psychic tables. (for example Biomancy AND Telepathy" but I am very confused about somthing. Say I am using Ahirman, I want to roll on Daemonology (Malefic) and the Tzeench powers, would I get the primaris from both tables? Or do I have to just roll on one table to get the Primaris?

Basically:

Roll on one table: All powers you roll plus the Primaries. (Psychic Focus).
Roll on multiple tables: All the powers you roll. When you roll for a power, you can opt to take the Primaris instead.
A Chaos Psyker with multiple powers? As above, but you get the Primaris of your God. (So Firestorm)
A Daemon Herald that rolls only on one a table, besides that of its own god? Example: A Herald of Tzeentch that only rolls Malefic Daemonology. All the Daemonology powers rolled, the Daemonology Primaris, AND the Tzeentch Primaris.

So with Chaos Space Marines, if they have the mark of a god, would they automatically get that god's primaris?

Check out page 22 of the rules. The Psychic Focus box on the bottom right.
Marked chaos psykers do indeed get the primaris for their god.