This bonus does not seem to double up to +2 projectiles if the player dual-wields Bargoll’s Cores.
DW Bargoll's Core
The relic is Mogdrogen’s Ardor for clarity. Was using it alongside the Mythical Voidmancer’s Cord, Cores, Mythical Will of Bysmiel (in ventory) and a +1 Occultist headguard (also in inventory) to reach 26/16 to Summon Familiar while testing.
20-25 Summon Familiar by base has 4 projectiles. 2 Cores should increase this to 6 assuming they stack.