Safest decision is to add only one of the tangs. I believe you will likely have success with one tang in your 90g, especially with the yellow tang. However, 2 tangs is really pushing it. Also, 8 fish in the tank, with 2 of them being large fish (the tangs) will really stretch the biological filtration your tank can handle. Could you get both and have a chance they will be fine in the tank? Yes, but it is a risk. I wouldn't go so strong as to say you CAN'T go with the list exactly as you have written, but I'd RECOMMEND not getting the blue tang.