Caster levels don't stack like that. He would be a CL 2 Paladin for paladin spells and CL 3 from sorcerer. If the sorcerer were to cast burning hands, it would do 3d4 not 5d4.
It's just like how channels from Paladin/Cleric don't stack. A basic rule is most class abilities do not benefit from other class levels.
#HeroLabFTW
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A spell’s power often depends on its caster level, which for most spellcasting characters is equal to her class level in the class she’s using to cast the spell."
Source:
http://www.d20pfsrd.com/magic/#TOC-Caster-Level