Broad Study (Ex)
Prerequisite: Magus 6, levels in another spellcasting classBenefit: The magus selects another one of his spellcasting classes. The magus can use his spellstrike and spell combat abilities while casting or using spells from the spell list of that class. This does not allow him to cast arcane spells from that class’s spell list without suffering the normal chances of arcane spell failure, unless the spell lacks somatic components.
As written, Broad Study only allows spells from the other class to be used. So if you were a Magus 6 / Elemental Sorcerer 3, you could cast your Sorcerer spells with Spellstrike and Spell Combat, but you couldn't use your Elemental Ray ability with either.
Would it be overpowered to do so? Use your Wizard's Arcane School ability with Spell Combat as a Magus / Wizard, or Summon Monster ability as a Magus / Summoner? Or if you were a Magus / Cleric, use Spell Combat with a Channel Energy in place of a spell. Something like that? Would it require a higher level version of Broad Study (with Broad Study as a prerequisite) to be "balanced"?
As it stands, there are things that are "as you cast a spell", like the Admixture Wizard Arcane School ability, which you could use with Spell Combat as long as the spell you were casting was on your lists, but there are a lot of "other class" abilities that you can't.
Arcanist Exploits? Lightning Lance vs. Metamixing? One is used like a spell to blast your enemies with lightning (totally Magus) the other allows you to add a Metamagic feat to a spell without increasing its casting time. As written, Metamixing would work with Spell Combat, but Lightning Lance would not.
I could have sworn it was spells and abilities of the other class, but that might have been a playtest version I'm thinking of. If doing so would require an "Improved" version of the Arcana, what requirements (other than Broad Study) should it have (if any)?
I would just make the change to broad study rather than having it be a seperate ability. I also think broad study should be available at 3rd level, but that's a discussion for another thread.
As far as your cleric idea, I made some NPC's who were whip users, Magus 6, cleric 4, Mystic Theurge 10. PC's got one hell of a suprise when that whip caught them with a harm spell.