if you merch holds the info weapon, then both of you benefit from the lvl12 conviction aura (-85 resists) but only he benefits from the -45 to -55 light res. which is only useful if he wears a dream helm, because in all other setups he does not deal significant lightning dmg.
if you hold the infinity yourself, then your char benefits from both the aura and the additional -lightres whereas your merc only benefits from the aura
long story short: if your merc does not wear a dream helm, -lightres on his infy is completely pointless and purely cosmetic