[Rendar]

Is there a possibility that, through a certain trait, or even a skill in mage (or possibly even talent. WHO KNOWS), that mages be able to combo invocations with one another?

This would likely only apply for invocations that are C rank or above, but the idea is. Person A starts the invocation. Let's say Shine knights.

Person A starts it up for normal cost
if Person B has the invocation spell; then cast at regular to advance the invocation by one 'chanting' round.
if Person B doesn't have the invocation spell; then cast at double or triple the cost to advance the invocation by one 'chanting' round.

This would lead for a lot of fun tactics where partying up with other mages can yield absolutely DEVASTATING results should they decide to work together. It'd also totally fit that whole cultist theme of people chanting together to summon an all powerful demon overlord--