re: Adding another return air vent

Well you can certainly go either way, and if a second return has to be placed in a location you rather not have, then go with the larger MB return duct, it should solve your problem assuming no other issues you are not aware of. But again remember after the new return air ducting is installed you should be able to to adjust the dampers in the ducts to hopefully better distribute the air equally to all areas of the home so that you have uniform cooling throughout.

Also try to keep doors open to all interior rooms, if there is no need to close them, which helps draw air throughout the house to the air return registers.