Game Mechanic Edit
Simple, you go into the armory. Next you choose two weapons you want to combo, they would be selected by some button. When selected you can go into a match, you double tap the fire button to switch to the other combo weapon and it will fire asap. Selected weapons must be part of the current loadout. This method of weapons switching is not exempt to any weapon switching rules.
This will require a nerf to weapon switching in general. Essentially this makes weapon combos viable, as it takes away a button required to press which is even more inconvenient on bigger screens. Now imagine toasting someone with a flame thrower and quickly switching to a shotgun to finish them off.