Proper use of 1 fireplace, when you have 2?

I have 2 fireplaces. House is a split level. One fp in the LR on main level and 1 on the lowest level, folks have called, the sub basement. I’m using the LR one but the lower level seems smokey. Does the flu, (sp) need to be open on the lower level when using the upper level fireplace? Can they both be used at the same time? Vise versa on using only one. Thanks.