Individualize 2 returns into plenum?

Hello! I have a 2 story home with a split central HVAC system (I believe that is what it is called, I have a thermostat downstairs and one upstairs) with one compressor outside and one air handler in the attic. I am getting estimates to make some repairs to some of the duct work in the attic due to moisture that has built up due to a bypass damper that is taken off the side of the trunk line instead of from the top. Thus, two HVAC companies have both said that the solution is to relocate the bypass to the top of the trunk line.

However, one of the companies also stated that there is another problem due to the fact that the 2 returns are tied together and then the bypass is introduced before tapping into the plenum. Aside from relocating the bypass to the top of the trunk line, they also recommend individualizing both returns (and have each of the returns go into the plenum separately, one on each side of it). The other company is saying that they actually don't recommend this (I can't recall their reasoning).

Can I get some input on whether or not it is a good idea or not to individualize the two returns like one of the companies recommended?

If you need more information about the set up, let me know and I will do my best to provide the details you may need.

Thank you in advnace!

source https://hvac-talk.com/vbb/threads/2235519-Individualize-2-returns-into-plenum?goto=newpost