I ran a 2e campaign with 2 players who were both invokers, that was a fun though it was a brief campaign. I think we started at level 9 though rather than working our way up from 1st. We did at some point add a couple of henchmen, a fighter and a cleric. Most memorable moment was when one invoker summoned up a wall of fire and caught the other invokers genie in it killing them. May have also caught the other invoker in it.