It depends how far is the air return register (the hallway one) from the actual furnace and where is the filter for the unit installed. Besides removing dust particles air filters protect furnace’s interior components / air passages from contamination. Having an air filter installed on a return register is extremely important if the heating system serves as an air handler for air conditioning coil.
So, if the current filter sits just before air enters the furnace (before the blower) and if the hallways register is close to that area that second filter is not essential. Of course, it would give you extra filtration power and protect section of the air duct between the register and the furnace. Air filters are designed to allow air to pass freely so you don’t have to worry about blocking air flow for as long as they aren’t heavily contaminated.
If you current filter has been installed on an air supplying duct (after the blower) you can mount one in the hallways return register and eliminate the existing one. I have a similar system in my home; my filters are mounted in each of the two air return ports and there are no filers on the air supplying side.