I already understand how overtime dmg/sec works for elemental weapons (Shock/Corrosive/Fire), but if I decide to switch between two weapons of the same element, such as the Heartbreaker (81.6 dmg/sec) and the Gospel (180.4 dmg/sec), both fire, and shot the same enemy, would the damage stack? Replace the other?
Elemental damage over time from the same gun stacks, actually. There's no difference for switching guns, using abilities with different elements, etc.
Each stack of DOT exists completely on it's own and will die on it's own, and more will be added if an elemental hit procs regardless of other sticks.. Best I can tell, there's no maximum number of DOT stacks currently active, either.