Question on whether I can put a tee in a 1/2" gas line.

Hi New to the forum. Hope someone can shed some light on my project. I am installing a new Gas, fireplace insert in my upstairs family room. I installed the insert directly underneath the upstairs fireplace several years ago using 1/2" flexible copper line. Pulled a permit, had it inspected etc... Not an expert but I have done this before. Just don't know gas line codes. So... I would like to tee off of the existing 1/2" soft copper gas line I ran to the downstairs fireplace instead of running off of the black pipe gas line about 40' away. I would use flare fittings and I would use 3/8 " for the new gas line to the new insert upstairs. . Anyone know if code allows this. Thanks much for any advice.