https://irowiki.org/wiki/Double_Bolt
long story short - it affects all casted bolts.
dood, you either choose to be a caster noob or a physical attacker noob.
if you go towards the autocast route, you're gonna rely mostly on soul link, use a Shadowchaser's Madogum,
you could also go towards tank autocasts via destroyer boots proccing comet for AOE over single target. which could be mid-game gear at around base level 100+.
If you wanna go autocast I think you'd fare better using a Shadow/Abyss Chaser.
but being a wizard is fun as a super/hyper novice, just get some fixed cast reduction gear (+13/14 Full Faith Crown, Shoes that reduce FCast or an A grade +14 Moonlight Paw, Hero's Grace)
alongside the appropriate stats for 100% VCT reduction (VCT stone reduction set on costume + stats),
and you just wreck stuff via rotating Gravitational Field (from the super noob Magic Shadow Set which also gives double bolt), Psychic Wave (from Freedom Rod), Lord of Vermillion, and Meteor Storm before going Hyper Novice.