I don't understand half of what you said ,ask whom ever you heard all this gibberish from what you should do/have.

I have seen furnaces which need to provide 5 ton of air flow (2000 CFM) installed with a filter on each side. At a minimum you would need a second 16 X 25 inch filter. However if your main return is only 14 X 25 inches then you will have a lot of noise and poor air flow. You will not get 5 tons of cooling if you have undersized supply and return duct work.

Sorry I didn't make the question clear.
Currently I have one return drop 14" by 25" which I was told is big enough to handle 2000 cfm.
The problem is that this drop is tied to the furnace through a 16 by 25 by 1" filter which is very small for a 5 ton unit.
I heard that sometimes in order to deal with situations like that, another piece of ductwork is installed between the existing air drop and the other side of the furnace, and then it's connected to the furnace with a second filter rack. That way the furnace has two openings from both sides and it's supposed to have better airflow.
The second option I thought about, is to install 25 by 20 by 5" filter rack (rated for 2000 cfm), horizontally in the return air drop instead of the small 16X25X1 filter that is attached to the furnace.
Which one is a better solution? If you have more ideas I'd like to hear.
Thanks again!

My house has two 20x25 returns (in different areas) for the 5-ton system. There is some whooshing airflow noise at both, I think the returns should have been larger 20x30. My mother's house has one 20x30 return for 4 tons, it's noticeably quieter but also is a variable-speed system and runs at a range of airflow rates, very low to higher depending on the required cooling/heating level.

The vast majority of hvac contractors have insufficient training in air handling (duct work,blowers and filters) which results in many poor air flow situations. The industry wide solution isn't just more training for contractors and techs, contractors also should pay specialist to design larger and more complex supplies and returns. If contractors and techs are bad off as I say how much expertise are you likly the find amongest people who don't even install air conditioning every day,much less specialize in air handling? You have repeatedly quoted the two biggest liars in the country,their names are I Heard and They Said. Are you serious about making alterations based on what people say here? You will get conflicting information so,how will you know who to believe and who to disregard? My advice is to pay a specialist to assess the system and tell you what if anything needs to be done. If you are hell bent on using free advice and doing work yourself here's my take. Erring on the side of installing more return than required has less negative effect on performance than mistakes in any other part of installation but is more than offset by positive results. The two negative results of oversized return is (#1) more surface area for heat migration into return air and (#2) imbalanced flow in parts of building. #1 can be controled/ minimized by well insulated duct. #2 can be remedied by installing a manually adjustable damper in return most likely to cause imbalance. In the case of 2 story building I advocate dampers in BOTH upper and lower returns to allow for seasonal change from heat to cool. Since you are doing the work yourself I highly recommend sheet metal over flex. Even some metal mixed with flex is an improvement, the more metal the better. There is no such thing as filters causing negative performance by being too large but there's a host of problems that can result from undersize filters ( I believe this is your core concern ). Now we come to the area that brings I Hear and They Said out of the woodwork and from beneath the carpet. Minimum of Merv 8 rating on filters. I'll present my case , Mr Heard & Said can present theirs and you decide which train you want to catch. There are instruments and tools for this but most techs don't carry one on their truck. Klem's quick budget test to determine if filter is restricting air flow. With blower running you should be able to remove filter without difficulty using two fingers hooked at each end pulling toward you. If cardboard frame rips , filter is either overly dirty or ,undersize. A secound return with filter or if you have a 20x25 opening with ample space behind, fabricate a V shape retainer inside to hold a pair of 20x25,20x20, 16x25 or like. It is of importance that air can not by-pass filter under any circumstance. It isn't uncommon to find odd size filter racks built into air handlers that requires special order/custom made $$$ filters. Adding insult to injury they are usually extremely undersize.. Find about this BEFORE installation begins and insist installer add a filter rack in return. Better still is to always request a large optional filter rack if and when filter is to be inside handler.

So preform Klem's test and add another return if indicated or just add the return,it can't hurt.

I appreciate the advice. I attached here pictures of the system. How would you run the ductwork to the new filter on the other side? What size of duct work would you use? Thanks!

I'm impressed by the metal ductwork,not many systems get it now days. I'd like to help you out friend but asking instructions for adding another filter is like showing a picture of some lumber and asking how to build a chest of drawers. To be honest I'm no expert with installing metal and if I were I seriously doubt I could tell you how in 25 words. I digress to I Heard,They Said and Chas Ross. What you have wasn't installed by amateurs, have them install a second filter before you mess around and put your eye out.

I am not a professional tin bender, but I will share my thoughts. I can't tell from the photo you provided, but it looks like you have clearance above the supply duct. If that is the true, then you may be able to to add a duct within the joist bay, make a 90 degree turn downward and to the bottom of the furnace, make another opening in the furnace and add a second filter. You would need to look at the installation instructions of your furnace to see if your furnace is set up to allow this. It is going to mean moving the wiring and plumbing out of the way.

Is there another supply duct going out the back of the furnace? The one in the front start off big and quickly necks down to a small diameter. It seems too small for 2000 CFM.

The other thing I noticed is the gas supply lines appear to be copper connected to black pipe. All gas piping should be black pipe only.