Currently I have a 1/2 in gas pipe T’ed off of the main 3/4in 'main
pipe supplying the stove. The oven is T’d off further down the line in what looks like 3/8th for about 3 in (long) into a flex. I am replacing/moving the oven (bigger and near the stove).
Would the 1/2 in pipe be enough to supply the stove and the oven or do I have to get a 3/4 T then split that into (2) 1/2 in pipes - one for the oven and one for the stove?
Depends on how far the pipe has to go and how many BTU the range and oven are rated at. Generally, as you are aware most appliances are 1/2" so you will almost certainly have to buy an adaptor fitting regardless. Here is a chart that should help you out. http://www.dsd.co.pima.az.us/BCodes/gas_distribution_standards2.htm
It is building code for a county in AZ but most municipalities in Canada/US are very similar so it should give you a very good idea for length of pipe and available BTUs. It does not list 3/8 pipe, but you could probably figure it out quite closely.
Of course, all of this is not relevant if you are going to get a licensed gas fitter to do it for you as he/she will check in their own code books regardless before they do the job… 