Splitting a Water Heater Gas Line to Connect Gas Dryer
You need to check with local code enforcement. I suspect they will want you to explain how you know your gas line will supply enough gas flow (in BTU/h) to supply both appliances simultaneously.

Then I think the code will require you to use black iron gas pipe in separate lines to individual cut off valves for each appliance. Probably you could leave the existing valve in place, use an adaptor, and place a black iron Tee there leading to two individual valves. Then at those point connect the light, flexible corrugated supply lines. 

I don't think code enforcement will allow the light corrugated supply lines to have a Tee in them.

There is a special type of (yellow colored and thicker) Teflon tape for gas connections. Also there is pipe dope designed for gas connections.