You're aboslutely right that they don't do the same thing. Isocort actually contains cortisol which I confirmed when I took my last saliva test. The results show a jump in my cortisol of 2 points per pellet 1 1/2 hr. after taking. I used Dr. Wilson's Adrenal Rebuilder, Vitamin and Herbal drops and even used them when I took saliva tests and never saw the cortisol levels even budge.
You can take them together and at the same time if you choose. Dr. Wilson recommends that severe patients take them both; Isocort and glandulars.
IF your cortisol is really in the tank (as mine was, almost Addisonian levels - in failure) taking the Rebuilder is a matter of choice. They SAY it gives the adrenals the 'cells' it needs to rebuild but after taking them for almost 2 years and seeing absolutely no improvement I decided almost $50.00 a bottle for the rebuilder was not worth it for me. Every body is different and each person responds differently depending on their level of adrenal fatigue.