Compressed Fire Bolt

By casting both firebolt and control fire, 2 or more people can create a compressed firebolt that, upon impact, does the combined main spell casting ability (Charisma, Wisdom or Intelligence) plus the level of all casters to the target creature. If interrupted the spell will combust causing from 2d10 to 4d10, depending on the DC save (Dexterity), fire damage to anyone within 25 ft. 

Each caster must have control flame.

From level 10, the spell increases by damage (1d4) every 2 levels, until it reaches 5d4 extra damage.