Will the perks of the weapon and its cells apply whilst using the other weapon, adding +9 to the total perks, or will only the perks of the weapon that you're holding apply?
And if its the latter, do you think that people will just stick with 2 weapons that provide similar perks for a more consistent build, or will having 2 unique weapon effects be more desirable than that?
The passive perks of weapons will generally be available at all times, like the rest of the aspects of your loadout, but there will be quite a shift in perk economy and what that means to go with this, so it doesn't necessarily mean adding an additional +9 to your perks.
To further elaborate on this a bit, the perks and benefits on Weapons will be almost exclusive to the weapons themselves, whereas the 'cell effects' you are used to at the moment will generally exist on and be adjusted through armour choices.