Multiple return air vents in single framing cavity
As with anything air is always going to take the path to least resistance. Return air is often overlooked by installers because it will work with just a big open duct next to the furnace. If the rest of the install is done fairly well, l then a poor job with the return mayl not have a significant negative impact on the house. If the walls are open I'd address it now. I actually spend more time on return air than I do on supply. If you have architectural drawings with a heat loss calculation they will specify the required cfm (cubic feet per minute) of supply and return. Then duct and return cavities should be sized accordingly. A single stud cavity is only good for about 120 ish cfm which may be good for two rooms depending on the rest of the system and the size of the rooms and if the doors are undercut, the list goes on.

This is a forced air system www.hometips.com

And this is how you would add an HRV to a forced air system www.finehomebuilding.com