So... say we have a magus, with Potent Magic. He invents a potent version of Pilum of Fire with a staff as a casting item, giving him a +3 bonus to spell (project a bolt or other missile). He then enchants his staff as a talisman and attunes his talisman for a +3 bonus to project a bolt or other missile.
When casting his potent Pilum of Fire while holding his talisman staff does he a) get a +3 bonus to casting or b) get a +6 bonus to casting?
b) +6
no reson why it shouldn't be cumulative.
No dissenting views? Cool. My players will be thrilled. 
They'll stop being thrilled once you blast them with one...
But yes, they add
Just remember that the bonus from Potent Magic is limited only by Magic Theory score, while the talisman is [strike]also[/strike] limited to a single bonus (the highest single bonus) [strike]as well as MT score[/strike].
Actually, unless it's in an errata I haven't read, I don't see anywhere that says a talisman's bonus is limited by Magic Theory.
Looks like you're right! It's only the number of different elements that is limited. I was so sure...
I'll just amend my post above.