supply ? for furnace

Hello, I have a simple question. Right now my house has two furnaces they where installed by the prior owner and not done well by any means. I have since had installed proper trunk lines for both and done well I might add. I am having issue with my furnace keeping up with these cold days. My question for the time being will it hurt to have the down stairs " in the basement" furnace that feeds the first floor of my house pull its supply air from the basement to help with added air flow? The return air I am assuming was just not big enough. I have the heat set on 71 degree and my temp on the first floor is not getting above 67. I know its more efficient pulling the return air from the rooms being heated but just now doing the job. It is around 10 degrees out side right now, so really I am thinking its more of just too small of a furnace for the area really and that I need to look in to getting a larger one later. Unless this helps and then I will need to have my hvac guy back in resizing my return. Thanks

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