Just had a Septoplasty on 7/23 with Submucosal Resection, was able to breathe in through nostril 1 day post-op; as of 2 hours ago feels unable to breathe in through the nostrils at all. Is this a cause for concern?

You must discus. this with your treating physician.Call ,first thing in the morning.Im sure you have had several days lately,when you couldn't breathe through your nose,with all the surgery and all.
Not really. After surgery the tissue are thin and the passage is wide open..after second day there will be more edema and swelling that will compromise the passage of air.so what you are experiencing is kind of normal,if you continue to be concern,check with the surgeon.meanwhile keep the ice,and sleep with your head up,and stay well hydrated.